Hi Jin,
All you really need is the Total Business.Net plan. A dedicated application pool will isolate your site from other sites and can help with loading times and caching the site.
You can point as many domains as you like at your site and the best way to do this is to domain forward the name from the domain registry.
You only need branded nameservers if you want to setup your own hosting company and hide your provider. Basically the multiple IP plans are for resellers who either have multiple ecommerce sites or who want to resell ecommerce plans.
We have AJAX 1.0 installed on all servers but you will need to load the control toolkit into your own bin folder.
With our new bandwidth offerings you should have plenty of room for the site to expand before you need to upgrade your plan.
If you have any questions regarding this please let me know.
Cheers,
Mark
System Administrator