So how are you going to be testing the payment gateway APIs that you choose to include? Live accounts or maybe you've got someone at each gateway who can set you up with a test account?

Also - a lot of gateways do include load balancing, so some people might be better off just going with their original gateway, instead of adding another layer of complexity (and costs).

But I definitly think you got a viable idea going here... there is money to be made, simply cause your startup is in an area which wouldn't exist if there wasn't any money being moved around.

Good luck.