Pretty much all covered by the rest of the posts but any way.....
M20 is one of my all time great engines. Strong, smooth and sounds wonderfull.
Depends what you want from an E30...to get it to drive spot on most if not all of them will require some work. Many will need much of their suspension changed because they wear and will have a major effect on the way an E30 drives.
Lots of them will have way passed their sell by date on sub frame bushes and shocks.Front arm ball joints and bushes will be common to, but not expensive. 40 quid for an arm and 10 or so for a bush, will make a massive difference to the way they drive if they have worn items on them.
Interiors tend to hold up well, always expect to find the dash with a crack in it though. Drivers bolster will be worse compared to the rest of them, but they dont wear to bad on the E30 partly because of the way the seat belt runs around the bolster not over it.
Engines are usually ok, check for cambelt change. Many will have already got through a head gasket and like most BMW's the cooling system isnt that great. Water pumps,thermostats and viscous couplings being the most common.
Rust and overall condition of body work is probably the biggest problems now just due to their age. Many will have had some work or paint at some point in their life. Rust on arches, sills and in the boots are common. Around the front window to I have seen many times. If it has a sunroof check that to.
Sports are the most desirable especially Tech 2's. Many fakes around, check the log book. Also, check the headlining. Sports were black. They also had LSD's, and Tech 2 body kits.
Buy a car with the orig BBS wheels, they drive a million times better than an E30 with 17" wheels.