US Dollar, hopefully can expand beyond US eventually, but need to start somewhere...
Billing will be invoiced monthly, with recurring payments (charges) against the card - I will not always have physical access to the card, so will need a remote billing solution.
I know Intuit offers a few, just want to know what others have used.
i'd look into chase paymentech or authorize.net if you are able to get a merchant account. chase's api is pretty blackbox but they have a soap api as well. authorize is less black box. Afaik, aka last time i checked, a while ago - chase was offering better rates and transacts in many currencies.
What api requirements/constraints do you have?