Well, the main issue is that your customers won't have access to dealing w/ CCK fields. These things would be better suited as attributes, especially since you'll be able to use the attribute system to alter the price of their subscription. So the products would be built and their options specified through the attribute/option system, but customer wouldn't necessarily use the add to cart form. You can make your own multi-step form that simply uses the Ubercart API to add the right product w/ attributes to the cart.