Nest one content type in another to be able to add single fields
That's quite a cool question...

I think If I were doing it, I would have the basic form available to all standard users of a standard permissions role...

Then when a customer has paid for the privilege to use more advanced fields, I set up a rule to promote their role to an advanced user role after checkout.

You can then use a module like Field Permissions to determine which fields your basic users can edit, and which fields your advanced users can edit.

And, you only need one content type.