It wasn't too difficult, if I can remember correctly, its been 10+ years since I dropped the motor in. You need to get the correct inputs to the ECU and the correct outputs to the dash. Get the two wiring diagrams and start splicing the wires. You can't believe how many extra inputs/wires the E36 wiring loom has that aren't required to run the engine. There are always hiccups with any conversion, but overall, it wasn't too difficult. There is also a ton more information on the web about the conversions than 10 years ago. I put in an E36 tranny and did an E36 steering rack as well. The only thing left from the E30 was the chassis and dashboard.
I had CDs from Zionsville that helped immensely for the basic conversion, ODBI conversion, and steering rack conversion.
We did two cars, my 1985 and the shop owners 1988 M3. His was published in
May 2007 Bimmer Magazine "Double Sixes."