Looking for method to allow selection of Pro Site level on sign up

Hi,

I am seeking a method to enable users to select which pro site package they would like on sign up (prior to completing the user and site sign up screens (and email authentication steps).

I understand the current system requires a user to sign up and set up a site prior to choosing a package. (The trial period can be set to a suitable package level to trial the offering.)

But is there any method to allow a user to select which package first?

In an ideal world I wanted to find a solution that allows the users to select between packages via a slider - where each increment is a new subscription level- but to the user it appears as a simple slider that they can use to select the package. Does anything like this exist already?

Thanks
Chris