This is a well thought out rack I think, a great starting point for the budget. All the bases covered for basic subtractive groovebox.
About the only thought I had on it was, maybe you shouldn't double up on the Synthrotek VCOs and Drums. My line of reasoning is, if you can diversify with a different but similar module that meets your requirements, why get duplicates?
So staying within your criteria of DIY kit, relatively same cost and HP, what do you think about the Befaco Even VCO - kit? I'm having trouble finding any DIY kit drums....all the Hexinverter Mutant kits are long gone :(