If you're looking at cobbling something together, then CPC do mini flat screen TV's cheaply enough. I picked up a 7" widescreen for 50 quid plus VAT a few months ago, though that was on special offer.
For the video source, I don't think you can go far wrong with one of these.
Freecom Media player 25 2.5" HDD enclosure USB2.0 (no HDD) available at Micro Direct Ltd UK
I bought one a couple of months back, and while it has one or two annoying bugs in the firmware (the main one being that it sometimes doesn't sort episodes into the right order) it's been a cracking player.
I'm in the process of trying to wire this and the 7" TV up in the car. I think I've got all of the bits sorted, so it's just a question of finding the time to do it. The Mediaplayer requires a 5v supply, but I got a cigar lighter to USB charger adapter from CPC for under a fiver. The only thing I need to do now is find the right plug for the back of the Mediaplayer because if you try to power it through the USB interface, it assumes that you're trying to use it as a USB hard disk and shuts down the player functionality
I've also worked out how to make a plug-in remote sensor for the Mediaplayer so that I can hide the main unit in the boot and still control it. If you want details of the bits needed, just give me a shout.
To be honest, I wouldn't bother with freeview in the car. It's hard enough getting a decent analogue picture on anything other than a massive antenna, and I reckon freview will be a non-starter on the move.