Also, is it necessary to use an eCart cart for the checkout in a site that is built with DataAssist or can you use a third party cart such as RomanCart ?
I just had a look at RomanCart and it looks like your cart would be stored on their server. I tried to look at the registration page but the server timed out and I couldn't get there. Not a good sign if you are going to depend on them for all of your sales.
It's a little more work, but having total control over your shopping cart has many advantages. The fewer outside dependencies, the fewer things there are to take down your site without your knowledge.