Matt is right, rebuilding a 3.6L will cost 13-15K. I think 200K is getting up there; knowing the service history and personality of PO is paramount. You must get compression and leakdown tests with that kind of mileage.
as with other e34's, suspensions, especially rear SLS may need lots of work.

be careful. without seeing either car, I'd suggest checking out the 80K one first.

study these:
http://bmwe34m5.com/faqs