Well, the M3/5's are really in another category in terms of price. However "used" bimmers move quickly so that if you want one now and trade it in for something - especially an M3 or 5 you won't take much of a hit on the depreciation. In fact, a Bimmer dealer in Vancouver BC is looking for newer used BMW's as there is a large market for them here and there just isn't that many of them on the lot.
My buddy and I are both looking for something in a 3 series, but not a 20, preferably in a 2002 or up, but would accept a 2001, and to be honest there isn't that much (in the colours and configurations we want). I have a 2003 Toyota Corolla LE with less than 20,000 Clicks on it (less than 12,000 miles) and I know here in Asian crazy Vancouver it will move very fast. My buddy has been given a good line of credit and is willing to jump when something comes in, but he has been looking for close to a month now, searching at four dealerships, newspapers and buy and sells.
Buy your BMW today, worry about your specific ride tomorrow.