I have seriously though about it
The gearbox from the M20 will fit on M50/M52 engines, but it will be at a different angle, about 20 degrees off IIRC. You'd probably be better of with a new box to match then the gear stick won't be at the wrong angle. The should sit in the right place though, plenty of E30's running around with M50/2 engine and gear boxes.
Personally my choice would be the M52B28 that Richard has in the compact. Starts out life with 190bhp. Stick an M50 intake manifold on (much larger pipes) and a BBTB and you looking at 220bhp. There's a chap that does kits. And a suitable rolling road map and you'll get anything up to 240bhp. Rock solid engine too.
I've seen the B28's go for less that 1k at times. Should be some E36's out there quite cheap these days - you can even get M3's for 4k.
In the M50/2 range I'd avoid the 2ltr its not got enough grunt really about 150bhp.