How to integrate payment processing with GWT / GAE based app?

I have a GWT based app, which is deployed on Google App Engine for Java. The app uses Google Accounts based authentication. I'm maintaining basic user information such as email id (from Google Accounts), last login date etc. in the GAE datastore. The access to the website is free. Anyone can use it using his/her Google Account.

Going forward, I would like to make it a paid service. However, I have zero experience with setting up and operating an e-commerce website. So my question may be a bit vague. I need some guidance on how to go about it.

Here are some of my requirements (but I'm flexible on the exact implementation):

1. Offer 2 different types of account - free and premium.
2. I do not want to maintain any credit card related information in my system. I would also prefer to not maintain an elaborate user database.
3. When a user first logs in, he/she automatically gets a free account.
4. User has to "upgrade" to a premium account to access all features of the application.
5. User has to pay a one-time fee to upgrade.

Given this information, I have the following questions:

1. Is GAE suited for my requirements?
2. Which payment gateway (Paypal, Google Checkout etc.) would be most suitable for my requirements?
3. What level of integration is required between my app and the Payment gateway? I would like to maintain minimal user information in my app. I want to focus on my application development and want to spend minimal effort on user administration.
4. Would I need to implement a custom authentication mechanism or continue to use Google Accounts or another OpenID based authentication?
5. What other things do I need to consider?

I'll appreciate any help on this. Thanks.