My quick and dirty opinion:

- Dotnet is new and unproven. Microsoft has a poor track record for the security of their web server software. I would be wary of doing it in Dotnet for this reason. This doesn't mean you can't make a microsoft web server secure, it just means it's harder to do and you've got to have the tools and the time to watch those servers like a hawk.

- It would be easy to do in Dotnet. Setting up the web servers and coding the application would be relatively painless. And yes, one advantage is that you could code it in such a way so that the web version looked and felt exactly the same as the standalone version if you wanted it to.

- If you're used to doing Microsoft development already, then there's no reason you can't do it with existing (pre-dotnet) Microsoft server technology. In fact, if we were doing something similar, this is the route we'd probably take because we know how to do it this way already.

- You could also do exactly the same thing in just about any other web server system other than Microsoft's. From your description, it's pretty simple: Getting data out of a database and displaying it in HTML is the basic bread and butter of any web server system.

- No matter which system you use, you'll need to do load testing before going live. Even after the load testing, there will be some growing pain when you do go live. Like you said, that many users in a four-hour space is a lot. Whoever sells you the server license, regardless of the vendor, is going to blow smoke up your yinyang regarding TPM numbers. You might want to ensure ahead of time that the system is scaleable even if you don't anticipate a need for growth.