Aaron,
You dont need a special plan to start with. You can start at the bottom and then upgrade if you need to. Tiat way you arent spending more off the bat.
OSC is fine, you might want to go to the OSC site and look at the contributions that others have made so you can start looking into who you want as your merchant. They have modules for most of the larger and a few smaller merchant companies.
Your own SSL can be purchased here for $85 plus a one time install fee of $30, off the top of my head.