The M42 came in the US market in the 91 model year (last year of E30 except for E30 conv). The M42 came in three versions but all E30 only came with a 5 speed manual gearbox.
-sedan
-coupe
-conv
The desirable model is the coupe with Recarro seats, OBC, limited slip, spoilers, basketweave alloys,etc.. Prices are rising on this car because it is considered an enthusiasts car (poor man's E30 M3).
The sedan has the same mechanicals (but I doubt many came with a limited slip) and a cheaper price. The sedan came without the OBC and check panel near the rear view mirror. It also came without the sport seats, spoilers, etc.. Steel wheels with hubcaps were standard on the 91 E30 318i sedan instead of alloy wheels. Their are some great bargains out there on 318i sedan's. I know, I bought one for $1.2K in Aug 06, one owner with only 118K on car. Car had issues and they spent a fortune trying to figure it out so they gave up on the car. Turned out to be a bad ECU. I had to buy a 91 318i sedan parts car (hit hard on pax side) for $450 to start swapping parts and figure out what was wrong. It was my gain because I now have a lot of parts. I do like the 4 door model better than the two door. Some E30 bottlecap alloys really dressed up the car, it looks so much better without the steel wheels and hubcaps.
-The E30 M42 conv ran for two years, 91 & 92 since the E36 conv wasn't ready so the factory sold the E30 conv, E36 coupe and sedan side by side. Conv tops can be expensive.
You'll find lots of 318i's FS on Craigslist, hope this helps.
If you live in the SF Bay area, also check out
http://www.baye30.net a great resouce for No Cal E30 owners.
G-Man