the problem with starting with a small case (as highlighted by @ModLifeCrisis) is that the modular you want may not fit in the case - so you will end up buying more cases (which are comparatively more expensive)
the best way to start a modular is to get an idea of what modules you want and which modules are needed to support those modules (yes you probably do need vcas, mixers, envelope generators, etc etc etc) add 30% for expansion and then find the case too fit
starting with a tiny case almost always means that many more compromises have to be made - it's your personal, self-designed musical instrument - do you really want to make it a compromise??????????????????????
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities