Nothing they are very stout and run 2k+ hours without issue. The only failure I've had on the water was a cheap stainless steel hose clamp, replaced all with Awab clamps which are much stronger than T-bolts. Second engine got me home without issue, that's what they're for and BoatUS for $180 a year is a no brainer.

Nothing, drive oil maybe, that is the only alarm that rings with any regularity.

I’m on typical fresh water lake (non great lake) Maybe a spare belt and decent wire cutters/stripers/crimpers with basic crimp connectors. Basic amt of tools.

2 batteries for 1 engine boat. 3 batteries for 2 engine boat.

Good anchor with lots of rope and correct amt of chain.

20 ft tow rope

I prefer near ski rope length, so say 75 ft :)