For 4x6 picutes get a Picturemate or Picturemate Deluxe. Don't even consider the Canon, Kodak or HP printers if you care about your budget. Their cost per page is like 70 cents a photo vs. the Picturemate's 29 cents a photo.
As for printing in a car - get a Canon PIXMA IP90 portable printer or a smal llaser printer. Laser's use dry toner and can handle the heat and harsness of a car. Inkjets will have a problem with the ink drying up in the print heads and with the print head getting out of alignment due to an unstable surface (moving car lol).
I'd suggest a Samsung ML-1740 if you really want a printer.