hey man by what I was reading seems like you have had no experience with drifting, if I were you with low or no experience with drifting, I would not try to drift in an M3 no matter how cheap I got it for.
But a couple of my friends drift and they have 240SXs. If you want to learn get a 240SX they are cheap cars for like $2.5k you could get a S13 aka 240SX before the 95 year mark. The 90-94 are best becuz they are cheaper. After you should learn by trial and error in the 240 chances are your going to crash you car so once your confident at drifting in a 2.5k car then you can move on to the M3. I would hate to c anything bad happen to such a beautiful luxuriously fast car.
The easiest way to start a drift in any car is to slightly pull the e-brake while your turning this will make the back end loose grip but as mentioned before DRIFT BIBLE IS THE BEST RESOURCE ON HOW TO. The guy teaching it used to be into racing and every1 would be so board to see him win cuz they already new that Keiichi Tsuchiya was going to win and so to entertain them he made the car slide and thus make the sport more interesting.
Currently they made drifting a sport of itself in the US its called Forumla D (for drift). Drifting was made for more entertainment but if you dont know how to drift I recommend you practice in a Nissan 240SX or a cheaper car.
Mastering how to drift will help you improve your weight transition during turns and thus slightly improve lap times by reducing your need to really brake but it is usually used b/c it looks cool. Well good luck and take care hopefully this information helped you.