The MVP version would be to use PayPal subscriptions, not any of their APIs. To create a subscription, you just give someone a link, and their account will push money to yours every month. If they want to upgrade or downgrade, either of you can cancel the existing subscription from your PayPal accounts, and your site gives them another link for a new subscription at the new rate. You can integrate PayPal IPN to record the payments in your database to have a billing history. All of this involves nothing more than copying some links and wiring up an IPN script someone else has already written for you -- under an hour.