Hello all, I've spent a bit of time lurking around the boards and I've got a fair bit of info and I think I've got enough info to go ahead with a build now.
What I do need, is advice on the parts.

I've spent a good while looking at different systems from nettops to barebones and I think the best way to get the system I want is to build my own.

CPU - Intel Core I3 2100T 2.5GHz 35W
Ram - 8GB DDR3 Corsair
Motherboard - Asus P8H61-I
HDD - Seagate 750GB 2.5" SATA
Case - Antec ISK 300-150W + 80mm fan to keep things cool
Monitor Lilliput 669GL 7" touch screen

Things I'll be using the car PC for are running mapping software, videos (not while driving ) OBD software, webcam for recording driving and a few other tasks.

My car is a 2002 Landrover Defender 90 and I'm using a 300W inverter.