Options for Power:
Repeat after me: inverters are bad, mmmmmKAY.
There are two very good solutions, depending on your needs:
If you will be having a low power system (like a VIA Epia), with laptop components (hard drive, dvd drive etc), then get yourself a case with a Morex 55W DC-DC power supply already in it. Then, get yourself over to Carnetix and get yourself their CNX-P1260, which fits between your battery and the power supply in the case.
If you will be needing more power (using a desktop hard drive/dvdrom etc), or a faster processor, then get yourself an Opus. This is rated at 150W output, but it seems to be very under-rated (unlike most PC power supplies which are grossly over-rated!)
If you need a screen, get yourself a Xenarc 700TS, make sure it has VGA input though. In my experience, the Lilliput displays, while cheaper, are not as good quality. As for putting it in the dash, some people have fabricated their own casings etc, it all depends on how good you are at fabrication!
Most of the "custom" ones you are talking about will just be fancy fabricated cases, often containing exactly the same components