there is no plain "you need x nitts" answer to your question. It will depend on your car and where you intend to mount the disply. I can tell you that nothing readily availible today will stand up to worst case bright sun conditions. The real question is how bad will the sun be in your particular install.
I believe my Xenarc is rated at 350 or 400 nitts but looking at it I'd say they may be stretching the truth a little bit. Maybe I'll break out a meter and test it one of these days. I'm satisfied with it almost all of the time but there are situtations where it gets washed out.
Also note that using a touchscreen is going to cost you here. It both reduces the amount of light transmitted and provides extra surfaces for light to reflect off.
My recomendation would be to get as bright a display as you can afford and to tint your windows as much as you can.
Good luck and sorry I can't answer your question more dirrectly.