whatever works for you! good that you are going slowly and not just getting 12u straight off though!
yeah cases can get pretty expensive - it's easy to forget that they are very niche products - not mass manufactured - so a lot of economies of scale that you would expect in other areas, just aren't there - plus most of the manufacturers are in the 1st world - so labour is comparatively expensive - same for modules to a large extent too
I tend to just buy rails and DIY power and some wood, but I have bought a couple of cases
I think the Mantis and LC9 are the best value built cases by a long way - Mantis if you have more power hungry modules, the doepfer if you don't (the PSU3 is a bit underpowered for the 9u - great in the 6u though)
"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