Hello,
You can keep both ASP and ASP.NET projects on the same machine. Combining projects can be done, but there is an added degree of difficulty when trying to integrate the two, and also passing data between the two. But it can be done.
The language is your choice; are you more comfortable using
VB.NET, which is more similar to ASP syntax, or do you want to go for more of a javascript/C++ style language in C#? Depends on your comfort level. They both accomplish the same thing in .NET, except for a few very high-level functional parts.
Brian