If you can afford the E46 M3 and know how to handle it, by all means, go for it. Its generally accepted that the E36 is built for handling, where the E46 handles great and has some power behind it.
We don't have the best classified section here, but go over to bimmerforums or some place and snoop around their classified section. If i was to buy an M3 i'd buy it from an old guy that babied it, or another enthusiast that isn't going to destroy it and make poor choices with mods and maintenance. Additionally, if you buy it from someone on a BMW forum, you can typically look back through their posting history and find out so much about the car than you would from a seller from autotrader.
And oh yeah, if at 25 you're buying an E46 M3, you're a rich kid, but thats okay because i'd assume that you're out of college and can afford it. In that case, more power to you.