I just swapped an '89 M20B25 into my '87 a couple weekends ago. EASY swap: use the ECU and harness from the M20B25, and it'll start up without any splicing. Some odds and ends do need work: 3 wires I think had to be spliced to get the tach and eco-meter to work.
Overall, if I was swapping only for performance I think I'd be looking at an M52 or better, frankly. If you intend to keep the car for a long time, the cost isn't that much different. Alternatively, I'd consider boost...it really depends what you're looking for.
1998 328i "M-tech"
2001 TDI Golf. 1300km/tank commuter. Boring, slow, cheap.
1991 P-car: 964c4 Let the porcupine jokes commence.
Gone: '99 M3 vert, '98 M3 sedan, '91 E30, '89 E30, '87 E30, '91 Euro M5, '93 6-speed Corvette, STI v8 swapped '98 Impreza, '73 2002, 2004 325iT