Will 20" HREs from a E60 550i fit a 2008 535xi?
If not what offset do I need for 20s. Also, will a stag setup work?

AWD models have high 40s offset, all RWD E60 requires mid 20s. You can only go from high to low not low to high offset unless you take it to a machine shop and thats if they can even shave it down.