Many metro areas have a specialist that focuses on starter (and alternator) rebuilding/repairing. When my Boxster starter overrun clutch was making noises the tech at the local dealer said when the time came to do something about this he recomended sending it to a rebuilder rather than replacing it with a new one. Fortunately the noise stopped and there has been no need to explore this further.
If the starter must be replaced I have no experience with aftermarket starters.