If you do go with Wordpress there are two (maybe three) options worth considering. The first is Wishlist Member: WishList Member | Membership Site Software. It costs around $100, but is a robust, good solution.
The second is S2 Members: WordPress › s2Member ( Membership w/ PayPal® ) « WordPress Plugins. It's free (unless you want to get the Paypal Website Pro add-on). Last I checked, it was the best free option available. It also seems to be kept regularly up-to-date (very important).
A third possibility is to use [AMember and sync it into Wordpress (here is the plugin: [url=http://www.amember.com/p/integration/new-wordpress-plugin/]New WordPress Plugin | aMember](http://www.amember.com/p/)). A friend of mine did this. I don't know how technically easy it is, but I believe it offers more functionality than either of the two plugins I mentioned above.