[vanwyk4257]

You shouldn't need to swap transaxles to install CV's. I know you can buy complete rear axles with the Lobro CV's installed for a "reasonable" amount of money. As for condition, I wouldn't expect any of the nadella shafts to last very long, so I would just plan on upgrading them on either one of the cars you are looking at.

The main "structural" areas to check for rust are the front suspension pan, rocker panels, especially near the jacking points, the torsion tube area, and the longitudinals below and around the spring plates.
The front pan and rockers aren't that big of a deal to replace and aren't a killer. The longitudinals and torsion tube can be dealt with but financially unless you can do the work right yourself you are better off finding a car that isn't as rusted.

There seems to be a surge in interest in the 912's lately, probably becuase the 356's are getting so expensive so people are starting to gravitate to the 912's now. Without pics and descriptions specific to each car I would say those prices may be a bit high. I've seen really nice rust-free examples selling in the $10000-$12000 range not to long ago, although the market may have gone up a little bit since then.
Take your time and wait for the right car to come along. Buying a car that needs a lot of work will result in quickly having far more in the car than you're likely to ever get back if you sell it.

Good luck in your search!