Thanks for your advice, this has given me some things to think about!
Maybe at first I should just focus on creating a small playable system. I would then get a 6 or 7u case and integrate modules that can do external audio processing later. Also thanks for the always necessary reminder about utilities. I started with more but of course they slowly got chipped away as I switched out modules.
I haven't bought my rails yet, but I had read about the difference in sizing so I was planning on buying lipless rails. These here are all intellijel format though according to what I've read. The Line In 1u tile will also work independently according to what I could find here: https://intellijel.com/shop/eurorack/1u/line-in-1u/
Thanks for all the other information about the Two59 and the DC coupled interface! I will definitely look a bit more into that and the possibilities with ChucK.