Ahh, du snakker norsk?
And that's about all I can say in Norwegian. And that was likely bad grammar. I can also say I can't speak Norwegian, and the word shark. And a few other small words here and there.
At any rate, any screen that has composite input will work, as the Pi outputs composite. Alternatively, if you can find a small screen that takes in HDMI (this likely wouldn't be that cheap), that would work, too. I found a 7 inch one by a quick search on the internet
here, and I guess it's not that expensive all things considered. But you would need to build a separate audio amp.
And as far as batteries go, you're going to want to go for lithium ion or lithium polymer batteries. That screen I linked you to is probably a 12 volt screen and when it says 12 volts it means 12, so you'd either have to have a step up regulator and 7.4V batteries, or have higher voltage batteries (or put more 3.7V in series. You can get ~14.8V by putting 4 in series, but this would have a pretty small mah rating) and a step down regulator to 12 volts (assuming the device can't handle 14.8V, I'd give it a quick go at 14.8 with a fuse in line to see if it'll work or not, but I would imagine it should if it's designed to work on a car, as the car battery voltage varies from just under 12 V to like 14.4V or so).
As far as input goes (you didn't mention it, but this is important), there are several small USB keyboards that can be implemented, and you could likely get a 7 inch resistive digitizer and find compatible linux drivers to get it working on the Pi. If there are no compatible drivers I suppose you could make them custom for the Pi, but that'd be a pain. Keep in mind, before you plan on putting some piece of hardware to interface with the Pi, make sure there is already a driver for it, or else it will be lots of messing around with stuff to get it to work.
Errr, yeah. Any other questions feel free to ask.
Also, what part of Norway are you from, out of curiosity?
Edit: I'd likely look to buy everything online, I know things in Norway are ridiculously expensive. I was talking to a Dane and Swede at the university, and told them that my friend who lived in Norway said she would go to Sweden to get buy cheap stuff, the Swedish guy said he would go to Denmark, and the Dane said he would go to Germany.
