 E39 540i is one of the best cars ever made, bar none.  With that said, there is one thing to watch out for when purchasing one, the cooling system.  Water pumps with PLASTIC impellers, PLASTIC expansion tanks that crack, Thermostats that fail, and radiators that usually don't last past 70K miles.  CPO is great, when it runs out, do-it-yourself and save a bundle.  This car is put together right and working on them (and I didn't until this car) is a breeze.  You will not be disappointed with this car.