While I wouldn't touch a salvage titled car with a 10 foot pole, if you are going to buy one there are several things you need to do:
1. Have an expert look at the car thoroughly so that he can tell you how extensive (and what type) the damage is or was, and how well what has been repaired was repaired.
2. Be certain that the car isn't a flood salvage, as you will forever be chasing down electrical gremlins and rust.
3. Understand that insurance may be difficult to secure (at least full coverage), and that financing will be impossible. This may not mean much now, especially if you can pay cash, but it will affect you when the time comes to sell.
4. Understand that the decrease in value will be forever. Even proper repair will never restore the value of the car, especially given the fact that there are a slew of clean-titled M3's on the market at any given time.