You're a wise boy to think ahead and ask questions before you commit to the car! And good call realizing that xi offsets are different from BMW RWD cars. Generally speaking, offsets of ET 35 - ET 45 will work well with E60xi's. This site will provide you with some good information on what OEM styles will work well with your 528xi:
http://www.bmwstylewheels.com/. Also look at E46 (3 Series) and E53 (X5) wheels because they will fit E60xi cars also.
__________________
2007 530xi, Black Metallic Sapphire, 6 speed manual, Sport package, Cold Weather Package, Premium Package, Premium Sound Package, Sirius Satellite Radio, MTech front bumper, Replica Style 166 wheels