Developing a Desktop applications in C# is great. Its not just for Web Apps.
WinForms are going to save you huge amounts of time. It really is a first class citizen when it comes to desktop windows development. If interop is a problem you can always use P/Invoke and COM object wrapping that VS does for you.
Done right, it will be a breeze to maintain and update when the client changes their mind on what they want.
Yes, the framework needs to be on the machine, But this shouldn't be a problem if they have ever used windows update. Nowadays what language doesn't come with a run time library. In VS you can create simple installers that include the runtime.
Of course if your gonna deploy to Linux and mono, your mileage may very.