I would think Realtors do more than what is covered in studying for a license.

Homeowners policies dont cover vacant properties.

The rental rate they are currently getting doesn’t sound like it will cover upkeep and turnover.

Do I need a full year of rental history to prove to the bank that it can cover its own debt?

Only you and your agent can make the decision.If you buy it,  count on finding unexpected problems that require fixing, and make sure you have the cash on hand to cover them. 

The income would only need to cover the lender's loan via DSCR, not the lender- and seller-financing combined.

Also how do I make sure all secondary damage such as gutters, siding, windows etc are covered and accounted in insurance claim.

Quote from @Jay Chin: You can stick with the original lease, but make sure it covers what you want it to, it complies with current law, you understand it completely, and you are capable of enforcing it.

Costs and any inspections, work, etc that are needed can be part of your offer to the seller or covered during due diligence if you don't buy it "as is".

This is a new home and the builders in our area need to be competitive so closing costs were covered by the seller as well.