Hi Groc,
Please read the posts in this forum under the Rack section this post: "Why your 6U x 84 generative rig won't work", that's very useful to people new into modular. Please also consider to plan a (much) bigger casing. Please also read other posts in the Rack section to get an idea of what other new persons to modular came across.
Please try to plan less "fancy" or "sexy" modules however more classic components (they might look boring, they do look boring but you seriously need them) like VCOs, VCFs, EGs, LFOs, VCAs, etcetera.
The Arturia Keystep is a good start to get a sequence into your modular. Don't forget to look at an audio input/output module. The Eurorack signals are not 100% compatible with the audio signals so the best way to go is for an audio input/output module. But if you really plan to go for an Intellijel case (take the 7U, not the 4U), then that should be fine.
Good luck with reading, investigating, understanding (check/ask here if any info required) and planning your modular system, and please try to read the entire posts in the Rack section of this forum. Try to avoid the "TL;DR" principle ;-)
Kind regards, Garfield Modular.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads