I'd recommend the NASA
Opportunity, or the attractively priced
Spirit, both in good condition, far outperforming expectations on the road!
With good solar shielding, fold-up lander capability, a 2.6 meter ejectable cruise stage, and a heat shielded aeroshell, the 2003 model is well equipped for the highway, and for off-road.
Gas generators, onboard computers and cameras, airbags, and retrorockets all come standard. Spirit and Opportunity have identical suites of five scientific instruments: a panoramic camera provided by JPL; a miniature thermal emission spectrometer from Arizona State University, Tempe; a Moessbauer spectrometer from the Johannes Gutenberg University, Mainz, Germany; an alpha particle X-ray spectrometer from Max Planck Institute for Chemistry, also in Mainz, Germany; and a micro-spectrum Analyzer from JPL. Additional packages feature Honeybee Robotics rock abrasion tools.
Price comparisons and safety specifications available on
http://marsrovers.jpg.nasa.gov and
http://athena.cornell.edu.