Hi lanehouk
I'll try to answer all of your questions...
1. We currently support PayPal (Standard and IPN) but will eventually include several other gateways.
2a. No. Each theme has been developed with a specific purpose in mind.
2b. Yes. You could use multiple installs of WordPress (one for each purpose/theme) or you could use the multisite features of WordPress to tie them all together.
3. Yes, almost. Vantage is the new Business Directory theme that will be available shortly. You can check out our
Theme Release Status page for updates.
4. No. Each theme has a separate "back end". Although, multisite would make "single sign-on" possible.
I hope that I have helped.