How closely can I make a web version look to my desktop version with .net

The first thing that occurs to me is that if you did go this route, it's likely to use IE-specific stuff, which always pisses me off, as I don't frequent platforms that have IE available. Now, obviously, it's up to you as to whether you want a browser platform or an IE-only platform, but it seems to me that it would be more beneficial to provide services more openly. Otherwise, if you're restricting it to IE, then why bother changing it from a standalone app?

Can a Windows server (or servers) handle the number of users we are talking about (without astronomical expense)?

Obviously, it depends on how hard these users are going to be on the servers. What I can tell you, though, is that it's hard to find a Windows machine that has more than four processors, and I'm pretty sure that nothing beyond eight exists at all. You can get Sun machines (for example) with up to 106 processors. Obviously, the cost of that would be astronomical, but the point is that the server could grow with the app load. If you go with Windows, you'll have to make sure that it operates in parallel with other servers doing the same thing. This might be trivial or it might be nearly impossible. I don't know, but it's something to keep in mind.