The harness is surprisingly easy. There's a ground strap from the motor, the positive connections at the junction block on the passenger side firewall, and that big honkin' C101 round connector. That's it - BMW engine swaps are great!
I haven't done an M44 upgrade, but IIRC to get the motor installed you'll need to swap the oil pans and the intake manifold. If you have a later model, the drive belts are different; you'll either need to make it all serpentine or all v-belts. Grab the steering rack while you're there - the E36 has a quicker ratio.
The biggest difference is the injection electronics. The M44 crank sensor is in the rear of the block, the cam sensor is slightly different. Unless you swap the lower timing chain cases it's best to wire the whole M44 into the E30, wiring harnesses and all. To do that you'll need the EWS stuff - DME, EWS module, ignition key antenna ring, original key and wiring harnesses.