I guess it depends what you want to do ecu wise, as to which T5 engine would suit. Squaretop 850/early p80 are the easiest to get running with an aftermarket ecu, but are quite old at this point and any engine you find for that price will need a refresh/rebuild
Looks like Blackman's garage is using a RN T5 (round top 99-05).
You will need to change turbo/manifold on either of these engines for RWD.
He also has a donor car for M90 and 6cyl whiteblock parts needed for the swap, reducing his project cost.
If you're not prepared to spend 10k on this project then it would be better to modify a redblock