Drupal Paypal Integration

I am planning to accept payments using Paypal on my Drupal website. There are no products or carts or anything else, just a user account with a credit balance. The user will manually add credit to it if the balance goes low, as simple as that. The important thing is the checkout flow, the user should be able to pay using his credit card on Paypal and be redirected back to my website. When the user is redirected back, his account should be updated with the new amount and should see either a success or an error message. I can handle all the back end code for updating the account after a successful transaction, displaying status messages, etc. I will just need you to help me with the Paypal integration. You can use either the Paypal module in Drupal or write code, I don't care. You should be able to show me your work so I can maintain and change it in the future. No matter what, the integration should be 100% foolproof. This is a new service I am starting and can't invest much so try and keep the amount as low as possible. If you can make it such that everything including the transaction processing can be done on my site itself, then I can think of spending more.