How would R-124 work instead of R-123?
This appears to be very similar to Isobutane (R-600a) with a boiling point of -11C (R-600a = -11.7C). Yes I think it would work, although it would be better suited for a water-cooled unit, since the higher boiling point of R-123 seems to work better at moving heat for the elevated temperatures in air cooling.

Being so similar to R-600a, it makes me wonder what the price difference would be between this and R-124.

I think what I like the most, its not even the tubing and piping which is nice and neat and compact, its that the compressor and condenser with fan assembly comes together, something I never get here in the USA it seems is those damn fan and condenser assemblies. Plus a nice cheap case!
Yes I totally agree. Much of the work is done for you, and it allows you to concentrate on the business end, that being to build the HX Stack. And as you pointed out, the main part of the case is already provided as an additional bonus. Just need to fabricate a front panel. Speaking of which, did you check out Front Panel Express? They offer a very slick service, and provide a free CAD application for doing the design.

I do like your heat loader coil Michael, could easily be stretched out a bit more and become an immersion chiller coil.
But at these temperatures, what would you use as the liquid?

Thats awesome Michael! Kepp working, i am thrilled for more pictures of the progress
Thanks godmod Don't worry, more will be coming soon.