Yes I have a touch screen on both my Mill and Lathe and also on the Mill I am retrofitting at the moment. I use 17inch TFTs and get a touch panel for them, they cost about £100 ($200) for the touch panels but I would imagine you would get them a lot cheaper in the USA, everything else is

If you go down to 14" or smaller you will definitely need to make up your own screenset as you will never be able to press the correct buttons with the standard screen, you could of course use a stylus but I find them unwieldy.
You need to watch what touch screen technology you choose, I have both resistive and capacitive and they work well but I did have SAW technology on an ELO screen, it was a very nice screen, extremely accurate but the drawback was that if coolant (or liquid of any sort) got onto the screen the cursor would go crazy which is very dangerous for a CNC machine

What I did was remove the ELO overlay and bought a resistive panel and put that in.
Heres a pic of the control panel on my lathe
Hood