customize the registration form

If you want to do it as easily as cheaply and easily as possible I think you could probably just buy:

1) User registration solution
2) DataAssist
3) Digital File Pro

You don't really need securityAssist unless you are doing something more complex with the login. You may want Validation Toolkit if you want to limit what size or type of files you will allow to be used.

If you know how to write php you can probably get away with just getting the solution.

In the long run if you are going to be customizing and developing other sites I'd suggest just buying the Super Suite and the User Registration Solution and you'll know you have all the tools and the functioning site that uses them.