Just because 2.1 is in beta doesn't mean you can't use it. Please, use it! It's ready to use, and the only reason it's beta is to let people know that it's the first release of a big update and hasn't been heavily used and tested by users yet. So there will likely be fixes before I release something official (non-beta). But there should not be significant design changes. If you have a project created with the beta, you should be able to continue working with it in the final release of 2.1.
As for a GUI, if you're going to run the game in windowed mode, the GUI could be accomplished quite simply by using Windows Forms developed normally with .NET controls, as demonstrated by the player options dialog box. You can see the code for this among the other code in the Source Code portion of the project tree. Is that an option for you, or do you want to run full-screen?