Just my .02, I wouldn't lease a loaner, or buy a loaner. They are beat on by everyone.
I've spotted people smoking in loaners. God knows what else they do in loaners. With BMW's big discount drive on now, I would think that the price diff in new vs. used is very small. Buy new with some peace of mind.
I don't beat loaners, but a lot of people do. Remember you're still paying a good buck plus tax and interest for someone else's beater. And the very good question comes up....what if you actually want to buy this car at the end?
Auf Wiedersehen 12 550xi