Before changing the starter I would take a good look at the battery cables and other wires associated with the starting system. Make sure all connections are tight and clean. One time I had a problem with my starter turning slowly and it turned out the battery cable connections were corroded. I cleaned them real good and that solved the problem.

Another time I had a problem where the positive battery cable got too close to the exhaust and some of the insulation melted as well as the wire was exposed to too much heat therefore creating extra resistance.