Excellent feedback, thank you! I'm glad you think that this project is not a fool's errand.
I had not considered rack widths beyond 104hp based on typical sizes I was seeing in commercial cases, but I do now see that 126hp is indeed an option for a more authentic 2-row layout. I do plan to build the case myself out of simple box-jointed pine boards or plywood, and already have 3 sets of 84hp rails purchased for the project, but I suppose that I can save those for one of the wing blocks ... you assume correctly that I have considered building those as well if this initial project is a success!
I have not otherwise looked at genuine spring units within the context of Eurorack, but I will do some research on that. Your talk of effects reminds me of my pedal board full of guitar effects (including some nice reverbs). I ought to consider an effects send-return that I could use to patch into that. If anything, that could offload rack space used for effects in favor of what I already have.
I do like the the look of that Olitronic Pro Out, and the 2hp MIDI module is what I currently have on hand, which is the only reason why it's in there. I'll need to do some research to better understand what this unit would and would not allow for compared to other interfaces with more gate/CV outs.
The sequencer solution will probably deviate from the original, in that I don't tend to write waltzes so am not sure how I would approach a 12-step sequencer other than setting it to run 8 steps and ignoring the last 4 steps-- a strange design choice by Roland indeed! I do like the idea of a sequencer "block," but want to focus on getting this first block done before I commit to others. Block 1 is daunting enough, as I'm DIY'ing all the Pharmasonic modules, have fairly limited experience with electronics and soldering, and this is my 1st forray into Eurorack.
Based on your excellent input, I've updated my design:
I'm hoping to keep the cost for the project to about $1400US for the Pharmasonic modules and case, which is certainly better priced than buying an original System 700!
Based on your