Mike,
I can't answer that clearly because the definition of "easy" is subjective; what is easy for me may not be easy for you and vice versa.
By no means is what your asking small in project scope, it is very in-depth and will required some thought before you begin implementation of any sort. What I suggest is going here:
http://demo.dotnetnuke.com/
Set up an account and you can play with a DNN install for 24 hours and you can get a basic feel of whether or not it will do what you need it to do.
In so far as your SSL is concerned, you will need to purchase a security certificate from Veritas (or any other company), install the cert on your webserver and also install SSL on your server; this is not an overly difficult task though it can get expensive (the certificate i mean). As far as the payment gateway, this normally is dependant upon WHO you are using for a gateway as it can vary.
-------------------------
I will only tell you how to do it, not do it for you.
Unless, of course, you want to hire me to do work for you.
^^Thats my signature
Help to develop new portal
