Forget about the price of the car (for a while) - and shop around to find the most solid bodywork you can find. The more rust you buy, the more you'll regret it later (guess how I know). Worry less about the motor. Motors are cheap compared to the ten or twenty grand you can easily throw at replacing rotten structure - besides, once the honeymoon's over, you'll lie awake at night wondering about Motronic, M5 motor and 5-speed upgrades, which are more interesting things to spend your money on than bodywork.