E46 M3 is a great platform to work with, you can buy it and leave it stock or mod it little by little over the years. I bought a e46 M3 and have been very happy (and poor) since then.
Another option instead of getting an E30 M3, is to get an E30 325i instead. They are almost as fast, come with a decently peppy motor and can be modded to be faster than a stock E30 M3. I think they are much easier to work on and there is a lot of support for that platform from specE30 racers. Just sayin', I want an E30 M3 too, but for $3500 you can own one that is pretty much there and you won't feel bad beating the snot out of a priceless classic. Also consider a Z3 coupe. Basically the same platform as an E30 M3 but with a better motor and more modern brakes and whatnot.