I don't quite see the benefit of buying an ipad or ipad mini specifically for this....but I think the project is very cool.
First, you have to buy yet another ipad, instead of being able to remove the one you have now and carry it with you, and
Second, if you get the phone circuitry, you have to pay your carrier another $30 per month for that device. (At least with AT&T I do.)
Maybe you're intending to make the ipad/iphone removable?
Better would be to spend the money on one of the new full-height head units available on Crutchfield and elsewhere that have ipad/iphone communication/control capability built in. This is the hot thing in the aftermarket right now, and there are several good choices already.
So, you get the best of both worlds: the ability to use your iphone/ipad apps on the head unit, AND all of the nice radio stuff (Pandora, etc), and be able to take your iphone/ipad with you when you leave the car.
All that is really needed then is something like the FIS-Control,
http://www.zzottel.de/index_en.html to be able to take as an input sensor information (boost, oil, water, etc.) and communicate with a display better than the FIS (i.e. the head unit or an ipad) and one can have an app that displays and logs all of the car information.
It's a great time right now with lots of cool products coming out.