Iylia,
Got the engine for cheap, previous owner put a 3.0 into his e39. That cost me about 2500
Original labour cost was 1600, but working out the bugs brought it up to about 2500. Good news is, they know how to do it now.
One of the major mods to do is the oil sump, which is reversed in the e39. (if the e34's sump ran down left to right, the e39 is right0left, cant remember which way it goes)
Should change the rear differential, didnt change the computer box. had to upgrade my brakes and exhaust, cos of increase in cc from a 2.0.
Soon I'll lower the front suspension, as the new engine is lighter than the original. Now the car looks funny haha.
Got some decent increase in power and reduction in fuel consumption.
Hope this helps.