A couple of years ago I had issues with my Merc starter, so I took it off, which was very straightforward and stripped it down for inspection.
As it turned out it just needed a good clean, which did the trick. However, as I understand, it was a bosch type and all the spares are available from a bosch electrical dealer.
the cost of a new one was at least £200 so I would say take it off strip and inspect before renewing anything. If you don't feel confident then take it to a bosch service centre and have them service it for you.