Mini Starter grind :(

Step one in ANY electrical diagnostic is to charge or replace the battery. Since you gotta do that anyway to drive it, and you gotta do that to properly diagnose it, that is what I would do.

A "Start" function on the charger is boost to a weak battery, not a dead one. If the battery doesn't have the juice, the starter may not have enough "go" to effectively shove the drive into the ring gear.

Also, it's tough on starters. It can weld the contacts.

Batteries are easier to replace than starters on most older vehicles...