All E30 (1984-1991) 325s have M20 inline-sixes; the i and is models have a 2.5L (M20B25), and the e and es have a 2.7L (M20B27). The i-spec motor has 168hp and 164lb-ft, whereas the e has 121hp and 170lb-ft.
While it is possible to have a 325i with two doors, most are sedans. The 325is and 325es are nothing but coupes, and basically the same as a 325i coupe with slight mechanical differences mainly in suspension and probably a different LSD (although the es still has a B27). I don't think leather is standard on any E30s, but it was an option on virtually if not all of them.
It's always nice to have the haggling ability you get from a private party, but with such old cars, I think it's safer to go through a dealer. At this point in a car's life, the price difference won't be that great, provided the dealers aren't completely evil, which may tuen out to be the case.
You could learn a lot from reading Spiff's thread
about buying an E30.
Welcome aboard, questions welcome.