The basic starter design has been in use since at least the early '80's. In almost every case I've worked on, the issue was the contacts in the solenoid, which pit due to arcing over time. There is also a copper plunger that is pulled in by the solenoid to bridge between the two contacts and it will also pit. Usually, there is one of the contacts that will wear more than the other. The solution in 95% of these cases is to replace the contacts...which I used to buy for .50$ a piece. I think there are several on-line sources where you can buy them for $5 or so for the pair.
In the majority of cases, there is absolutely no need to replace the starter. In fact, I'll bet a million bucks that most of the "remanufactured" units out there are simply cleaned up starters with new contacts installed.
BTW, I've never worked on an ND starter where any other parts, including brushes, bearings, or bendix drive. Realistically, you should be able to get about 300K miles on the hard parts and replace the contacts every 100k to 150k.
This guy has some of the best prices!
Starter Contacts