Here are some pics of the system I installed last summer in my E38 ('00 7-Series). I bought the car cheaply because the original BMW satnav / screen / stereo was broken, with all the components loose in the boot when I picked the car up. These bits were sold on Ebay, which paid for most of the bits required for the new system.
The PC I am using is an old HP Vectra - these were a mini-desktop machine that bizarrely ran 18v DC. It's low-spec by today's standards, but does everything it needs to, and was a nice size and shape to fit into the space where the original BMW computer used to be. I'm powering it with a cheap Chinese DC-DC converter which cost about £12 - I didn't expect this to last very long, but so far has been fine.
In the photo below the PC is of course supposed to line up with the slot in the panel. Somehow it ended up in the wrong position last time it was installed, but I wasn't about to move it for the sake of the photo:
The rear-view camera is mounted into the plastic cover that clips into the receptacle for the detachable towbar. I'm in the process of making a fibreglass panel which will tidy up this area and fill the ugly hole that was left in the bumper when the towbar was fitted.
Here's the screen itself - after a LOT of work I was able to squeeze it into the panel that housed the original (and much smaller) BMW screen. It's a tight fit inside there, believe me. The panel is hinged at the bottom and folds forwards for access. Behind the panel is a JVC headunit which performs all the amplification and radio duties - the PC is simply plugged into the line-in input on the headunit (and the TV tuner is plugged into the AV-in input).
The touch screen controls have been mounted in the lower right position, with three new buttons above them (in retrospect I could have done a neater job with these). Of these new buttons:- the left one operates the relay which provides power on to the TV tuner, the diversity antenna and the PC's power supply. The middle button turns on the rear-view camera, while the right-hand button is a momentary-type which turns on the PC itself (with the 'power' and 'hard disk' LEDs above it).
The top slot on the right allows the display on the headunit to be seen; obviously the buttons on the headunit cannot be reached with this set-up, but that's not a problem as the headunit's remote control works fine.
Now for some shots of the screen in use - apologies for the poor quality of some of the photos below, but the only camera I have is the one on my 'phone.
TV (Songs of Praise!). The TV tuner is a stand-alone item which is independent of the PC. There is a Kenwood diversity antenna stuck inside the rear window. Reception is reasonable but not great. This is the only in-car TV I have experience of, so I don't know how it compares:
PC in operation, running a RideRunner with the standard skin:
PC Navigator 10. Works well - much better than Autoroute 2010 which I had on previously:
Visualizations. I've not managed to get these working properly inside RidRunner, so I have to play music straight from Windows if I want to see these:
Lastly, this shows how the headunit's display can be seen alongside the touchscreen. I know this approach seems a little low-tech, but it works well: