If you search the forum you'll find answers to all of your questions. To save you some trouble, the big concern with 545's is the coolant pipe that runs through the V in the V8. It often develops a leak that can cause oil dilution and/or fry the alternator (if coolant drips onto it). Factory fix is very, very expensive ($5K+) and an aftermarket fix (with an expandable replacement coolant pipe) is also available that most reputable indy shops can install for ~$2.5K total. I'm sure some 545's never develop this leak and will run for 100's of thousands of miles, but you'll probably hear horror stories from some members and may decide that the 550 is a better choice, despite the higher cost.
So look for leaks -- coolant and oil. Also, have the oil checked for the presence of coolant -- if any is there (even a tiny amount) you are probably heading for big trouble.
2008 535i - Monaco Blue/Tan Leather, Sport, Premium, Cold (+ heated rear seats), Navi, HUD, PDC, CA, Sunshades, Fold down rear seats, CPO, Extended Maintenance