I can think of three options thus far:
Rely on a third-party product for your touchscreen that has a simple and platform independent implementation. Best example in my opinion is the Amulet touchscreen. You design your GUI in HTML, and use their widgets for user-interactivity. You just have to implement their simple serial protocol to be able to transfer data to and from the touchscreen.
Though I don't have personal experience with these options, I did look around a bit and saw an open source library called eGUI. If you want to try it, there is are some old downloads out there... it looks like it is not really actively maintained, though.
Do it yourself. I usually lean towards the solution that's ready to go out of the box, since the company can justify adding on a $200 touchscreen that offers all of the capability out of the box. However, since it seems like you don't want recurring costs, you might want to roll your own. I don't know if you need all of the features that emWin gives you, like VNC support.
Here's another related question: https://stackoverflow.com/questions/366278/graphics-library-for-embedded-systems-without-linux