Hello Sam,
Yes, the last time I was reading geekhack regularly (about a year ago) I recall your efforts, very interesting stuff!
As you've noticed, I'm not fussed about the base unit or the interface used, as I'll set that aside and put my own controller in to scan the matrix. I don't plan on destroying any hardware (we are talking about pretty rare stuff).
I haven't tried the Model F switches, although I plan to get a 122-key terminal board (do a controller for that too if I need to, although many seem backwards-compatible) to sit alongside my 122-key Model M. I'm attracted to the beamspring not only because of the reports of the wonderful key action, but also by the very cool cosmetics etc. (I love spherical keycaps, for instance; you can't get those for a Model M or F!).
I'd love to be able to find a Displaywriter keyboard for $30 (don't like my chances though)! Although, they are probably the least interesting cosmetically. On the other hand, the layout looks like it could be practical as a daily driver for me (with a little reinterpretation of keylabels of course -- I have a job as a `squiggly brace' language programmer).
The absolute maximum I could spend with good conscience would be $US100, and that would be high, because I still have to ship the heavy bloody thing to New Zealand, and US shipping costs are very high these days. However, I've obviously got what the rangefinder camera guys call `GAS' (Gear Acquisition Syndrome), so if something shiny and pretty comes up then who knows.