Hi Martin.
Regarding the donor tx, if you want to install it just for the time you will be rebuilding your original tx....
If it looks OK from outside, and you don't see any visible oil leaks I'd leave it as is. You can replace the o-rings on dipstick, pressure control cable and modulator valve, these are easy to replace. Pull the oil pan and inspect the condition of the oil, if it's clean then just replace the oil and filter.
I'd check the modulator valve and pressure control cable to make sure there's no vacuum leaks and I'd do it at the ends where they connnect to the engine to ensure rubber connectors are goos as well.
I can't tell you for sure, but if you have an older tx there are a number of upgrades to be done, i think B2 piston is one of them.
Good luck