bigger is almost always better - at least in the long run - I would recommend 6u instead of 4.5u or whatever these intellijel cases are...
1u looks inviting - but the amount of functionality you can get into the row compared with 3u is very limited - and there is very little that is available only in 1u - that's not available in 3u and takes up a lot less hp - and the advantages of the built in parts are not really that great compared with the price difference imo
I'd recommend a tiptop mantis - better power supply and cheaper than the intellijel 4u/104hp - and the 'built in functionality' of the intellijel can be added in not many hp - as and when you actually need it
if you want to use a guitar with eurorack modules I'd suggest that you get a dedicated input device that at least has envelope follower and comparator included = or get a disting mk4 or ex - as they have an algorithm that has both envelope and pitch following
an expression pedal interface module is also really handy - so you can control the modular with your foot whilst your hands are occupied playing the guitar
fx aid is a really useful module that adds a lot of variety in effects (and other areas) - the xl has both better ergonomics and more modulation inputs
"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