I went down this rabbit hole after buying a 0-Coast in October. I bought a 104hp powered skiff (also from Make Noise) and started ordering modules here and there. I started with Mutable Instruments stuff (which is such a common path as to be cliché) Rings and Clouds. Rings is a fascinating and unpredictable synth in its own right, Clouds—ignore the hype—is a bizarre and inscrutable effects unit that is either inaudible or smears your signal with something gunky that's neither reverb nor delay. Both of these units have cost me hours and hours of productivity as I grappled with both their peculiarity and with learning modular in general. It's clear that Mutable Instruments products are well-designed and well-made, but they are intended for a user with more modular knowledge than I had.
If I had to do it all over again, I would start by adding utilities that interact meaningfully with 0-Coast and understand the theory before moving on to "luxury" modules. I would purchase LFO/envelope/function generator, clock divider/multiplier, and voltage utilities (mult, VCA/attenuator, random voltage thingies).
It's only now, after abandoning the very specialized Mutable Instruments stuff, that my understanding of modular is growing and that the 0-Coast is playing more of a part. The more you know about modular, the more you realise 0-Coast is a spectacularly good deal for the $$$$ and contains a ton of functionality. Forget the fancy stuff and focus on learning the theory and basics.