As in what parts should I get to replace while drives/motors are out? Is there a comprehensive kit, and is any aftermarket bearing etc as good or better than OEM Merc?

Im blocking off the thru drive exhaust system with the Merc part number block off plate on the inside, is there a way to block it off on the outboard side with a cap or something where the exhaust bellow would normally be also?

you can put basically a rubber plumbing cap on where the front of the exhaust hose mounted, but unless you plug the 2 exhaust vents in the bottom of the transom shield, it won't help. not really necessary. the block-off plates take about 15 years to rot through.

Exhaust vents?