Cable condition as stated already along with cable size, 6 volt systems work best with 0 or 00 size cable, 12 volt conversions can go up to a 1 or maybe 2 gauge if everything is in good shape. Battery ground should go to the engine block then the frame and cab, block first to insure the most voltage for starting. You can not have to big of a battery cable, positive and negative should be the same size.
Starters are not complicated, pull it apart and clean it up yourself. Brushes and rebuild kits should be available online, simple tools, multimeter, and shop manual will get it done.
