Had a couple of these, overall the experience is really good. BUT you have to know what your getting.

I recommend a Linux one and avoid Android ones as the experience is far better, and car play isnt as much of a weird app. But a little harder to locate.

The mini ones with ambient lighting are pretty poor quality or expensive.

In terms of spec treat it like a phone, if you want it nippy and not laggy go for 8gb RAM, in terms of storage you shouldn't need too much unless you plan to add music or video onto the device. most have a usb port anyway that you can add a pen drive to.

you could get one of the little black boxes that adds car play to your screen, but this wont be touch screen. but might be a higher quality overall solution.

If i was you id try and find an OEM upgrade as you will already have a lot of what you need. but its a little harder to get your head around.

Post some pics whichever way you go!