Not sure about the offset... but you def shouldn't need longer bolts, you should be able to use regular bolts for any wheel as long as you are not using spacers...
I would just test fit the rears and see if you will clear the breaks... how fat of a tire are you planning on running in the back?

__________________
'06 X5 4.4i| (08/06 built date) | Executive Edition | Sport Package
'08 X3 (wife's ride) - Sold
'05 325i - Sold
'03 325i - Sold
_______________________
projecttechnik.com - Alberta BMW Forum