if you don't want to create a personalized self-designed musical instrument then you are probably better off with a poly-synth - not a modular!
I started out with a absolutely tiny 72hp 6u case - within 6 months it was almost full and the next module I wanted wouldn't fit - so I bought a 6u 204hp case (mantis) and moved everything to that - when that was full - i used the original case too - and when that was full I built myself one, I now have 4 diy cases - but if I'd bought the larger case to start with I may still only have 208hp - not 1500hp - as I wouldn't have run out of space when the module I wanted was too big!!!!!
"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