I would check all leads are tight on battery and engine.
It sounds as though your starter is the trouble, the turning over slowly shows that is is engaged, pretty sure they are pre engaged on these, but the grauchy noise could be it not engaging correctly and chewing on the ring gear.
Either way it needs to come off, and make sure it throws ok, when energised.
If it is a BOSCH starter, it will most likely be the solenoid at fault, as the varnish/laquer comes off the solenoid piston and jams it in the housing so it doesn't reach full travel.
To fix just use some knackerd emery cloth about 800-1000 grade with oil clean inside of the solenoid housingh and the outside of the piston assembly that moves.
As to why it would turn over slow, I would have a good look at the brushes, see if they are worn or the starter is oil logged, if needed undercut slightly with a scraper.
tabetha