dude, whats the mileage on your current motor?
its seems very stupid to pay the extra cash for a 328 if your just going to rip out its heart for an s50/2
if your really serious about this i would suggest selling your car and picking up a high mileage 325, you could get one mad cheap, and if your upgrading the suspension and brakes anyway, the mileage wouldnt really matter.. as long as chassis wear wasnt to bad.
Just seems like your doing this a lil ass backwards. Your call though.