If you want to use C, I'd recommend GTK+ for widgets and windows (unless it's been removed in later PRs to cater for the Qt steam rolling).

You can develop on the device if you want to. If you think you need a more friendly environment, you can develop on your Windows box using a cross compiler and testing the binary on the device.

Learn to use SSH to log into your N900 and get to know how things are organized. There are good Wiki pages to help you out there. This is good to know when it comes to organizing whatever is to be installed on the device.

Since I don't know much about C bindings for Qt, I'd say Google is your best friend for finding examples or just general information.