Re: PayPal-integration ?
I think if you find a basic but good Paypal plugin for WordPress, that’s half the battle. The API is a bit awkward to work with (in my experience).
Have you talked to the WP e-commerce guys (http://twitter.com/danmilward) about bringing e-commerce into BuddyPress as a group plugin? I saw Dan present at Wordcamp UK and he seems quite a friendly chappy.