You only need to solve one problem: find a source to purchase Topre domes, springs, and sliders.
That's why an already-assembled topre keyboard will be handy for! You know, to salvage 
Nice! I didn't realize there were so many folks ready to burn money salvage parts from Topre boards! 
(I am aware of Crick-Cracks, KRAYMACKs, etc etc)
The Type Heaven comes in at a little under $1.50 a switch, that's not so bad. As long as you're okay with only 45g.
Maybe there's something cheaper that you could get from amazon.jp with a proxy, I'm not sure.