S50 cams are often $250-300 on Bimmerforums. Just be patient and keep an eye on the classifieds.
As far as the impact, you'll have to get a very good strong one. My impact wasn't able to get the crank bolt off my S52 block. It's not that strong of one, but it is rated for 300+ ft lbs (it probably doesn't do that much). Btw, what are you using to hold the crankshaft in place?