You can get a helluva good deal on a 99-2003. Thats your best price catagory with relatively low milege.
The 525 isn't a fast car, but it corners very nicely and is a nice highway cruiser. You'll get a min. of 250,000 out of the engine before it starts giving you any problems. The 1992 525 that I drove in 2000 is the model that convinced me that BMW is a great car and people buy them for something other than a status symbol. This particular 525 had 190,000 miles on it with no previous mechanical difficulty other than normal wear and tear items such as brakes, rotors, etc. It drove "tight" enough to shock me when I realized it had a ton of miles. Prior to looking at the odometer, it felt like it had no more than 50k on it...
You can't go wrong with that straight-6 M54 block, its been around longer than the "BMW is crap" whines.

If you're looking for newer, consider a Pre-certified. They'll have a warranty up to 100,000 miles from the dealer if its 3 years or newer.
Parts will cost you 20% more. Dealer work on them cost about $1/hour more than Ford does here ($79/hour at Ford dealership, $80/hour for BMW dealership)