Here is a link for a DA Converter that will work with 12 Volts....
http://www.apogeedigital.com/s...nidac
I used Apogee products years ago when I used to compete in car audio competitions. It's good stuff, not cheap but good. I couldn't find a current company that made a actual 5.1 processor that would take digital in. 5.1 has not taken off as everybody had hoped.I have a Rockford Fosgate Dolby Pro Logic II processor that I am not using that works better then average for a analog device. That would probably be your best bet. I will let it go for $200.00 shipped
As for a center, if you are going to go to the trouble and expense of a outboard D/A converter then you would want to make sure that all three front speakers are the same model and size so they have the same "Voice". It will be a challenge to get a good 6" speaker in the dash or center console area and still retain even a moderately stock appearance.
it's too bad that you are stuck on 5.1 because if you do the math, the path length difference form right to left at the kick panel location is pretty close. That means that the Q should sound (Image and stage) pretty good with a couple of decent "Beamy" speakers mounted in some well designed custom kick panels. That is my plan, I will be using a pair of Boston Acoustics new SPZ60's in custom kick panels with probably a pair of McIntosh amps to run the system with a custom sub array in the rear deck. Hopefully it will sound great and hopefully yours will too.
Good Luck Scott