Kind of a newbie question, but I'm bored. Stay the F*** away from Audiobahn. That's my 2 cents.
I'm not sure that there are a whole lot of amplifiers with optical inputs available for the car. Although Im sure if you look around the board you'll probably find some threads concentrating on digital amps.
The PG is probably the best choice you've got here, but why screw yourself with a cheap amp? Not only are you going to ruin the quality of your sound, but you're also not going to have something that will give you any longevity. Stick to the good stuff. You get what you pay for.
Also, amps with a ton of channels usually start to slack off in quality. You just can't get all the component for 5 or 6 channels into one amp. You might be better of spending your money on 2 or 4 channels now and saving up for the next 2 channels on a separate amp. You'll get a good quality amp now and a better set up in the long run.