Hey,

The first place that I would point you to is the following:

http://www.asp.net/community/projects/

This gives examples of how web sites can be structured, especially the Beer House Sample kit, which is a very good example of a multi tier application.

With reference to your particular questions:

1) This is certainly possible, but how are you maintaining your codebase? Do you have access to something like Team Foundation Server? This application makes it very easy to combine as a team in different geographical areas.
2) Performance comes down to really how much data you are trying to display and correct usage of it. Given that you are familiar with T-SQL you should be able to construct the database and queries in an optimal way
4) Yes, this shouldn't be a problem. To start out, you could use something like Paypal, which there is an example of in the link I posted above.

Hope that helps!!

Gary