Well first of all you want to keep in mind that a case that small isn’t going to have that many power connections and you have like 16 modules. 2hp modules are great. Especially for the size. But you’re not gonna want to fill a whole case up with them.
As far as your question about buying modules on eBay when you see a good deal; I would say don’t. I don’t know what kind of music or sounds you’re looking to get out of your modular synth, but always keep that in mind. Also I would highly recommend buying a complete synth voice first. That way you can hear it, patch it, understand it better and really figure out where you want to go from there. That’s the best way to decide the next module or modules that you’re going to need.
(I had to learn this next one the hard way)
I would never recommend buying any modulars based on looks or hype unless you’re absolutely sure that it can help get you where you want to go. Or else you’re going to be selling it and losing money. Get modules based on what you ‘need’ or modules you think could really inspire or push your modular music further. My motto is that I like simpler modules. If it has a steep learning curve, then it has to be completely worth it to me or it’s a no go.
Anyways long story short. If you don’t know already, figure out exactly what you what want your modular to do at first and then expand from there. And do a lot of research. Like a shitload of research before diving in. It’s all very very worth it. But if you come into it unprepared, it can be a less the fun experience.