Where to start with the starter?

[quicksandfarmer]

So an update, hopefully to bring closure:
Over the summer the starter became increasingly unreliable. I examined it closely and the pinion gear was rounded off, it wouldn't engage the flywheel reliably. And when it would it wouldn't disengage reliably. I tried locating a new pinion gear but couldn't find one. Finally, I bought a new starter. Everything is happy now.