I’ll take a stab at this, since I went through these sorts of questions 6 months ago.
I am also writing from experience with Magento Community Edition, and I don’t how much it will differ from Magento Go.
Pre-Authorize Users: Nothing like this built-in, but you might be able to find an extension or write something that does this. Check Magento Connect for this and other extensions you might need.
Without extensions, you could hide all of your products to customers who are not logged in. Then you could move all registered users to a specific group that has access to all (or some) products.
Profile Pages: Built in.
Integration with external DB: Not really possible. Magento has a pretty intricate database spanning nearly 400 Tables. There are a number of tools out there though that will help with importing products into Magento.
Stock Management: Built in
User Groups: Built in
International Pricing: I don’t have much experience with this, but I believe some functionality with this is built in, and is managed by different store views, I believe (A magento installation can handle many stores and websites within it, and share settings between them)
Generated SKUs: Best solution is a Configurable Product in Magento, but you’ll have to build the SKUs yourself.
Keep order opened for a day even after you logged out and then send order to proceed: Need more information here
Customized Pages: Yup
Easy to Manage Backend: Magento Backend is pretty straightforward