You pretty much have it all - except for the SSL, which I would recommend and then you use a gateway that would actually offer you an API so you can integrate it into your shopping cart to have a seamless checkout procedure.
As far as pros / cons of each, there are always going to be some. There are tons of threads in the forums about choosing a merchant account as well as a gateway.
Each electronic payment gateway has some great qualities and some bad ones. But whatever you choose, also consider Paypal in conjunction with your cart
If you go with a host that offers fantastico you have the choice of three great shopping cart script. Oscommerce, Zend, Cube Cart. You can auto install these with the links provided. All you need to do is add your contact info and items. You can use Paypal to get started right away and then add you merchant account when you get it set up. Of course you can modify the graphics as well.
I personally use oscommerce but I like some of the features of the zend cart better.