Thanks for both your replies.
Pro forms doesn't have subscriptions that add a ccap (I can only find buy now)
I cannot have 2 pro forms on 1 page, 1 for PayPal and 1 for Stripe, so I could toggle with a tabs control or something
This requires PayPal Pro
This requires a user fill out all their details first.
PayPal has the users details it would seem, so all that's needed is a button to PayPal. That's very easy for the user. The return page then would create the user instead of the user manually registering. All its asking for is to choose a username which I'd like to override with the email address.
I'll have a dig in the code and see if I can do this in the success page. Any pointers as to where that code is would be handy
The 'thank-you' page happens after the registration when returning back to the site?
The custom params needs a pro form.?
How would I fork the registration system - or is this just a massive can of worms!