Really don't get the disproportionate reverence for the E30 M. I'd blame its inclusion in every other game, but then it doesn't seem to have worked with the Lotus 49, which is infinitely more interesting to my mind. (Heh, I wrote 42 at first and couldn't understand why it felt wrong. Come to think of it, I'd prefer the 42 over the 49!)
It's a great car that's rare as hen's teeth, and the first of what has become an iconic and very successful line of sports sedans. As a homologation special, it was much more race car than the later M3s, and reviewers tend to go on about how great it is to drive, regardless of it's age. It's a fantastic drive in Assetto C
If you've got the cash, you can just buy anything from an E36 up. But even if you've got the cash, buying an E30 M3 is difficult. Chances are most people have never even had the opportunity to drive an E30 M3, and never will.
It's one of a small group of cars that are very interesting both as drivers cars, and historically because of their effect on car culture. It's by no means a must have, but it is a very odd omission from a game with so many cars, particularly one that already includes a substantial number of BMWs. Instead they chose to model the 507, which is a
beautiful looking car, but is nothing particularly special to drive, and according to Wiki drove BMW to the edge of bankruptcy.
As far as BMWs go though, I'd put the M1 right up there for inclusion as well. Great drive, again because it's a homologation special, and historically potentially more important even than the M3, because it started the M-range thing. Rarer, more interesting as BMWs only mid-engined car produced in significant quantity, and I think awesome looking. But that's just me. I'd be happy with either.