the simple answer is to go slowly - it is not a race to fill the rack - it is a journey
buy the absolute minimum of this that you think you need in order to make sounds
I think you will want filter(s) and other effects
2 sound sources is as many as I would want in this size case, Morphagene would take you up to 3
I would go up to a standard 84 hp or 104hp case to make more space
"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