ok, that's helpful.
My plan is to put a 7805 on the board and power from unregulated 12V. I'll have to look up the opamps I picked so far, but I'm pretty sure they were TI rail-to-rail opamps, 2-3$ for a quad. a 12V and 5V on the input for buffering, and a 5V on the output, to clear up any input impedance problems.
The input opamps are there mainly to minimize loading the car's circuitry, I'm probably going to drop the 5V input buffers, since there is one on the output, and I don't think there's going to be a need to divide 5V outputs from the car (therefore it could just be piped into the output opamp). That's going to save me just enough room (having a hard time getting it to an economical size).
Thanks for the info.