You know you'll have to figure it out a way to turn it on with your car, right? There are many solutions available, just search on the forums.
Also in my experience laptops have much longer boot times, so you may want to use (and test extensively) resume from hibernation/hybrid sleep. GPS might not resume from hibernation, etc.
Lastly, due to the limited number of USB ports (a mini itx can have 10 on board), a laptop can have two, more on the docking station - but if it's not a native docking station, but a universal one, one USB port will be having all the traffic load...this can be a problem if you add too much.
Having said that, many have used laptops/tablets, and I think for the limited use you want it's perfectly doable (no USB cameras, USB relay boards, OBD, etc. etc.). Just do your research and buy from what works from people in the forum. See who elses uses a dell, what brand of gps they use, drivers, etc., that's a start.