welcome
I'm building a BMW sportster and have played with wheel sizes, see this thread:
http://www.madabout-kitcars.com/foru...read.php?t=557
8J's seem to be fine on both the back and the front with the right offset. 17" also look good.
Take a look here there's quite a lot of finished sportsters in the stoneleigh gallery. Ford ones have parallel front wish bones and the BMW's are at more of an angle!
http://www.msportster.co.uk/detail.asp?cat=12
The light blue sportster has a bigger overall diameter wheels (225/45/R17) than most of the other BMW one as its based on the slightly newer BMW 3 series that had bigger wheels. I estimate it to have atleast 8.5J wheels on the back going from the stretch of the tyres and 8J on the front but im not 100% on that as I didnt speak to the owner.