To be honest, i don't think it's that much more work to go with an M52TU or an M54 compared to the older single vanos engines. you still have most of the same problems to overcome, namely:
- EWS delete
- coding out DSC/ASC
- coding out emmissions stuff (post cat lambdas, secondary air pump etc)
- either coding out or wiring up clutch switch/brake switch to the ecu
- Speed signal to the ECU (either code out completely or use an E30 9 tooth signal and code ECU to accept that)
- RPM signal for tach - ECU should have a wire for this already
Most of the above is just removing OBD error codes to stop the ECU going into limp mode without emissions and DSC related stuff. Then if you go M54 you'll need to wire up an electronic pedal, but this is just 6 wires between the pedal and ECU. Or you can convert to cable throttle body and code the ECU appropriately (not sure if you can do this with an MSS43? never looked into it myself.)
The link below covers putting an S54 into an E30, so obviously all the pin numbers are different but it outlines all the same issues that you'd face with an M54 into an E21 (from a wiring and ECU coding side of things at least). There are a couple of M54 swapped E30 threads around but i can't find them right now.
http://www.r3vlimited.com/board/showthread.php?t=275042
), can you explain a bit more what is needed to be done to make the single and double vanos work respectively?