I'm surprised your agent is not supplying you with referrals.

Weeks later, the buyer wanted to make another offer of $13K off the asking price verses 35K off the asking price, agreed to the correct HOA Fees and wanted all of the furniture in the cancelled contract.I explained to the buyer that a new request to reactive the cancelled contact is required or a new contract has to be sent over.

If you buy for cash in an area like Sandusky you will lose capital since supply outstrips demand meaning you won't get any meaningful apperciation.

Turn off all the supply valves. 2.

If you are providing supplies, how will the inventory be managed?  

They only sell Carrier and Daikin, which are fancier than the currently installed Goodman unit.Is there any real savings to be had by purchasing the unit directly from an HVAC or plumbing supply house?

Well, besides PITI, there's maintenance which is basically the amount you allot to take care of things like replacing a door lock that someone used a pliers on, broken glass, door weather seals, HVAC maintenance, supplies to maintain the lawn, termite treatment, replacement boards for the fence, caulk and paint, etc.

I am doing all of the work of finding the deal, arranging a PM etc, and my investor is just supplying the funds for the down payment.

If you have a furnished rental, your maintenance goes from systems to furniture.

Yeah, I have the same issue in Austin, TX trying to find multi-family properties - little to no supply and very expensive.