My personal suggestions (which others may or may not agree with):
To keep your steering wheel controls working, you'll most likely need to keep your headunit. There are a few companies that make Aux-In cables for stock head-units. I got a cable for my '04 Grand Prix from Pacific Audio for like 60 bucks. The PC audio plugs into that, and my steering wheel controls control the volume.
I think one of the toughest things is figuring out how and where to mount the screen. You'll want to start pondering this. The rest of the PC can be hidden almost anywhere. My PC is in the cubby of my dash. Lots of people go for the trunk too.These things don't use a lot of
power, unless you build a system out of an old Pentium-4. Extra batteries shouldn't be required.
Software is the other big question mark.. Lots of software can do a lot of the things you mentioned. There's no shortage of choices here.
You should be able to do OBD, GPS,
Music,
Video,
WiFi,
BlueTooth, and rearview camera without too much trouble.
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