in a case this size I would really only want to try to get a single voice in, more than that and you can easily end up with significant issues that once that case is full are only solvable by either buying another case and adding more modules or removing modules and replacing them
it's significantly easier to start with a bigger case - work out what you need to support what you want and then if you want a small portable case - either additionally or as a "beauty" case for travel - you have an idea of what you will be able to fit in it and still have a working synth that you enjoy playing with
I would look at a mantis - great case for the money and still portable - unless you are a small child
if you are worried about spending too much too quickly on modules get some blind panels to cover most of the rack - they are not expensive - 40€ covers over half a mantis - and set yourself a budget - stick cash in a jar or something similar!
with the palette cases you are paying a lot for some buffered mults, 1/2 a midi interface and 1/2 an output module (maybe 1/2 an input module too) and the 1u row (which in terms of hardware costs the same as the 3u row)
personally I do not have any 1u and do not intend to - I would rather build or buy regular 3u rows - for example a mantis cost about 1/2 the price of a 7u/104 case - by the time it's full and there is need for what would be the 1u row - you could buy a second mantis (with the savings from buying one in the first place) and buy equivalent 3u modules instead of 1u tiles - and probably only take up 20hp or so - leaving 180hp free for other modules
"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