Layout you can easily change as you learn more through direct use, so don't worry too much about that. Availability is an issue right now. Mutant Brain is hard to get hold of; Steppy slightly less so. Make sure you won't be frustrated by learning using Mimetic Digitalis and one channel of MIDI-CV from Disting (plus what you can get from your semi-mods). I'm not sure that the sound of Chainsaw fits with the kind of music you listed in your original post. It is tempting because of the paraphony, but then you need enough pitch CV to drive it (not many in-rack sources right now, and by the way, pitch CV is the only reason to have Link 2, otherwise cheaper stackables or stars suffice). You might consider Instruo Ts-l (which has a wavefolder, so maybe Bifold won't be necessary right away). For mixing, I would suggest, instead of the two Mixups, a knob.farm Hyrlo and a Doepfer A-138s. (If you have off-rack effects, knob.farm Ferry is quite useful, but they say November at the earliest for that one.)
My experience was that my design changed not only several times before I bought my first module, but also as I started acquiring modules and learning about them. Think about the initial purchases and how they might best inform your future choices.