Please keep usability of the screen itself in mind.
For a touchscreen you would need to either a) let the OS run in very low resolution if the display is small/mid sized or b) find a different touch method like a touch compatible pencil for higher resolution. Otherwise using your fingers could lead to accidentally clicking on things you didn't want. Or just make the needed buttons really big and keep the other things small like it is. There are several methods of making a good experience with a touch screen but it can be tricky sometimes. Just wanted to give you my advice about this because I had a touchscreen project before where the concept was easy at first and it got more complicated because of the touchscreen.
For the restrictive part: You could restrict the UI itself or play around with elements overlaying parts that you don't need or don't want to be clicked.
The hardest part would be the keyboard: A virtual keyboard is set up easy. You could still use a physical one but that would make it more complicated.
But here comes the annoying part: A virtual keyboard takes away lots of space.
Which is why I thought about attaching selfmade buttons left and right (and maybe left and right at the back of it to not make the device too wide) next to the touchscreen and give them the keyboard mappings that are needed. Also combining several pressed buttons together can "emulate" a physical keyboard too, depending on the amount of buttons.
Creativity can solve a lot of problems here and can make it more fun working with your pi! Have fun and good luck!