Perhaps buy a Pod64x case and some cables, and ask your colleague who’s into modular if he’ll rent you some of his lesser-used modules. He might have fun picking a few out for you and showing you the ropes.
Going from nothing to a Shared System seems like quite a leap.
I know. Feels totally irresponsible 
But good tip, crossed my mind just to ask him if he has a couple of modules lying around and put those in a case together to take a test drive.
This is a good and wise choice. Resist the temptation to go from “no modular” to “I’m an amateur instrument designer.”
This is huge. In my early modular years I went the route of building something on Modulargrid. Researched the heck out of each module, cracked the code of what I thought would be a unique synth voice. It was awful and unplayable. It wasn’t the modules fault - I ended up keeping and loving the Mutable Instruments Warps, for example, it was a matter of not knowing how the modules would interact with each other or sound.
I repeat, to anybody considering this - software first, 0-coast if you must have something physical.
Totally agree!
Another thing I’ve just thought of is that combining multiple manufacturers; has the added complexity of potentially needing to offset to make them play nice together.
So if you are starting small I’d suggest getting modules that have the same voltage spec. So either all modules that expect:
-5V/+5V
or
0V-10V
Perhaps you have a sequencer that outputs one or the other which may push you down a particular direction?
If you have an OT then the minimum modular can be as little as one oscillator. @cuckoomusic has a video showing how OT can generate CV signals. You can let OT be the amp/filter/effects. Plaits would be a great choice for the osc. You can play with it in VCV to try it out.
You can also get a midi to cv module to get even more integration with OT.
Some starter modulars:
OT is known to be able to handle euro levels directly but you might throw in an attenuator just in case. It will be useful regardless.
This is mostly true. I mean I feel like plugins can’t get close to the real sound of certain analog polys or what you can make when you have that raw electronic power in your hands, in the physical world
But vcv rack is an incredible learning tool. I learned how to use sequential switches to make my own sort of “wavetables” or my own modulation sequencers. I learned how fm works, I even learned how the typical subtractive synth layout works with envelopes to filters & vcas with the vco at the beginning and the clock driving everything. I learned how turing machines work, learned to use marbles & stages.I learned how to make physically modelled strings with a delay and a filter plus a noise burst exciter using karplus-strong. And reverse-engineering the plaits modes taught me so much about physical modeling synthesis as well. You have all the elements you need in Rack
I wouldn’t have known what type of filter I wanted, that you could ping filters and phasers, what was going to drive my sequencers, how much and what type of modulation I wanted, what I’d need in terms of vcas or attenuation/attenuvertors and logic
Even the mixers in rack teach you about sends and returns with cv control for potentiometers. It’s a much greater one-to-one analog of what you’re going to encounter in a hardware setup than what you learn from a daw
You learn what you need. It’s like an interactive modulargrid which is pretty damn accurate in terms of sound and functionality. You realize what kind of tools you want for what you expect to build out of eurorack. and even more than that, you learn what is possible. You can learn teletype in there, you have a morphagene emulation, Erica synths, Befaco (muxlicer rules), all the mutable stuff, and much much more, all for free. You can get Maths for free, even
It’s worth a shot, if nothing else. Just install and play with it once. Take your modulargrid rack and build it in vcv to see if it works. Even if the exact module isn’t in there, I promise you’ll find something that does the exact same thing. I highly recommend grabbing every free module from the plugin manager & firing up an omri Cohen video
But I also did have an 0-Coast before I bought my little niftycase. And that did teach me that I needed a function generator that can do audio rate modulation (or a looping envelope). But I don’t see it as very useful for making music with. That’s just my own bias though. It is very cool and well thought out, obviously. But I think a Behringer monosynth typically is more useful.
I’m a big fan of the crave, but I never owned the mother 32. There’s a lot of patching to be done there, but you can’t do all the crazy things you’d expect from eurorack. It is useful for leads and especially bass. Such an amazing sounding analog monosynth
I see vcv rack as a sort of more interactive, easier to visualize version of max msp or pd. You can build whatever you want with more building blocks than you would ever know what to do with. Then, the cool part is that you can then translate all of those skills and tools into the real world once you become a millionaire
100%
In my case, I would have never known how useful most tools were, specifically Warps, if I wasn’t able to use it in Parasites in Vcv Rack before I bought the mystic circuits clone. That is hands down my favorite hardware effects tool that exists and now that I’m not using eurorack anymore, I’ve been searching for something that can do everything warps can outside of modular for a while
I also don’t think I would have any idea what to do with something like o_C had I not learned in vcv first. It would have been a much steeper learning curve for me
I started with a DFAM + Eowave Swarm and loved patching between the two.
Both of them sound great, which was important for me, since i wanted to use them in my tracks as quickly as possible. Also they both have sequencers 
Just to reinforce what you already seem to be doing and what others have suggested: think carefully about interfaces. While most modular gear is going to have common elements (ie CV control over various parameters coupled with knobs, etc.) the interfaces and actual use in a patch can vary a lot. For me, that’s been the biggest challenge for finding stuff that I actually like working with. For example, at present in my system that’s meant that Make Noise = No; ALM/Busy Circuits = Yes. For complex modules with deep capabilities, a screen (even a small one) is a huge help – but lots of shift functions with corresponding blinking LEDs and so forth? Not for me. And don’t assume that because everyone seems to be using a particular module that somehow that means you’ll need one also, and that it’s going to work for you.
I built up a pretty substantial Euro system with two Tiptop Mantis cases and a 104HP skiff, all stuffed to the gills with multiple sequencers, sound sources, modulation sources, random sources, etc. That’s since been paired back to one Mantis case and a NiftyKeyz keyboard/Eurorack combo after selling off probably 50% of my modules. I thought something random and generative would be fun, different from typical subtractive synthesis, etc., but in my case that turned out to be the wrong approach. I wanted/needed something I could actually play, and my current setup is turning out to be a lot more fun.
Again, if you have any way of trying things out at a local shop, even if that means just looking them over in person to get a real sense of size, placement of controls, etc., that’s hard to beat.
Sounds like solid advice, thanks.
I’m gonna hang around in my colleague’s studio tomorrow. He has a massive system, been building it for years and years. Gonna hear what his advice is and maybe do some patching of my own. I am really in no hurry, want to do this as thoughtful as possible (while I keep fighting off the GAS 
)
Beyond the 1, it’s okay to acknowledge that a fully modular setup may not work for you!
I can go in far too many directions and while I still have a small rack with modulation sources, I prefer to use Elektrons or semi-modular synths over the insane amount of flexibility.
Limits are good for me and my workflow/decision paralysis.
Let alone that the cost of modules is going to be vastly more than semi-modular synths.
I’m already diving in again. Today before work been noodling with Omri Cohen’s version of the shared system. 
Just ordered an 0-coast. I was tempted to buy the System Coupe, but eventually it was too beatboxy and sample based for me. Had to be honest, not follow my GAS.
My colleague actually tried to talk me out of eurorack, while I was standing in the middle of his 2000hp dungeon. And, he said, if you do not care to listen: start small or with a proper semi, just to see if the patching and the way of thinking clicks with you.
Also went to the local modular shop today, and the story was: I have shit all available, the semiconductor-crisis and the strong dollar are only going to make things worse coming year. He could provide me with a lot of (excellent) Erica stuff, but that is not the sound I am looking for (can get a lot out of my DB-01).
The only thing now, lingering in my mind, is a Fusebox X. But that (or eurorack in some shape or form) will be something for next year. Or not.
Good choice. It’s got all the essentials right there.
Just patch the hell out of the 0C and keep in mind what additional functions you’re really feeling, move on and then when you have two or three of those functions in mind, start thinking about a single-row rack.
0-coast or any other semi modular is a good start but if you want i made a rack based on the Loumavox , the “fake” synth Skiff add on - Eurorack Modular System from holografik on ModularGrid
I’m here slightly too late to also recommend the 0-coast, but I missed mine so much I bought it back after filling most of a 7u 104hp case. Sequence/sample and effect it with the OT.
It’s the business!
Yeah, really looking forward pairing it with the OT!
Very glad I chose the 0-Coast eventually. But I am not sure this eventually will lead me away from the dangerous eurorack path 
First try, using the OT’s arp and some of its delay and verb.
Okay, I liked the 0-Coast só much, that I returned it. Because I needed the money for this…
Perfect gateway drug that 0-Coast, oh man. Sold some of my other gear, annihilated my savings and now building a (blend of) shared systems.