I think that a little more clarification about the process you wish to implement would help us on the forum to better determine the correct way to go for you.
I'm just not very clear on the PayPal signup page part. Normally you would offer some product or service for sale on your site and the user would add it to their cart then proceed to checkout. Whatever options you make available will be selected before the user add the item to the cart.
It sounds like you would like to just adjust the return page that they are sent back to by PayPal based on these options for the product, is this correct?
Also, are you using Payments Standard or Pro? How you pass along info differs in these two implementations.