I also have a 2008 528xi... Mine came with OE Style 246 wheels. They are 8x18 et43 front and back. After lowering my car, I hit a bad pot hole and bent my front wheels. At the time, I was running 5mm spacers up front and 20mm spacers in the rear. I found a perfect set of the same Style 246 wheels... except they were from a 528i RWD, 8x18 et20 front and rear. Picked them up for $400, no tires, but perfect condition. Figured since I'm already running 20mm spacers in the rear... I took them out... 2 of the new et20 wheels in the rear w/o the spacers... and moved the rears to the front. Fits perfect, even being lowered 2" on BC Racing coilovers. I've measured, and planning to get VMR 710's in 8.5x19 et35 fronts and 9.5x19 et33... 235/35/19 and 265/30/19 tires. They should fit perfect. Just make sure what ever wheels you get, you can run staggered, just make sure the rolling diameter & circumference between the fronts and rears are less than 1% for AWD. The VMR wheels and tires I mentioned are -0.8% difference.
These 2 sites are perfect for calculating for offsets and for tire/tire stretch sizing...
https://www.1010tires.com/Tools/Whee...et-Calculator#
https://tiresize.com/calculator/