Ubercart

I'm currently working on bringing file downloads to core. Here's my thoughts:

1> Displaying all possible prices, via a product attribute, everywhere would require some custom coding beyond the scope a file product module. I'm not 100% familiar with uc_product module, perhaps it could be implemented via a product node theme functions? You'd have to ask Ryan or Lyle on that one.

2> Once the file download module is brought to core you should be able to do this. You can create an attribute, named something like "number of downloads", for a value of 1 you create a price of 0, for a value of 2 you create a price of -1, and so on. In the upcoming file download module, there will be a hook created that you can call to determine whether or not a user can still download a product. Based on the attribute value chosen, this hook could limit how many downloads a customer gets.

3> This would be out of the scope of the file product module. You'd need some custom module to keep track of product nodes (and attributes) a customer purchased. Based on that, you could have the module disable the add-to-cart button.

Perhaps you could elaborate further what is being bought and rented? I'm thinking perhaps our current uc_roles module might be a possible solution depending on what you're trying to accomplish.