If your problem narrows to the starter solinoid,it can be replace ,or
fixed,without replacing the motor.Check the  solinoid for a complete
circut.If you get one, the solinoid may be fixed,without replacing it.I
think they hang up cause of corrosion ,and get to much current, and then
burn out.
 After removing the starter,use a impact screw driver(advance auto $18)to
seperate the solinoid from the starter motor, proper. Remove the solinoid
and clean the piston and cylinder with 800or 1000grit sand paper. Grease
the piston and cylinder.Use RTV when reinstalling the new or cleaned
solinoid,to prevent moisture from corroding the new fix.Cheap, and not hard
to do.If you got the afternoon free,you will be very educated in starter
motors by doing this operation,insead of just buying a new motor.It is very
interesting how Bosh has put this item together.Very simple.2 bolts hold
the motor to the engine.Three screws hold the solinoid to the starter
motor.Thats all there is to it.