I don't know of an ac unit on the market that can cool below a 25 degree differential.  

property is currently in a reverse mortgage need $308k to satisfy. currently 3 units and wanting to remodel to create a 4th.Rural low economy market with high demand and very few rentals available in area.

They often have off-market deals that might fit your criteria better.

It's been on market for about 180 days. 

That at least puts it in perspective for me as I determine how to break into the market.

The plan: I am identifying on/off market properties that are targets, will do all of the real estate work (listing, showing, advertising, vetting, etc) and will split the cost of the purchase of the home and the material costs for any needed repairs.The contractor will be subbing out portions of the job and be on site mostly full time until the project is completed. 

will 3b has a steady market?

You might be able to get a better yield by finding a value-add project or a BRRR type property, but the market rate stabilized is 4-6% probably.Just be careful of insurance right now, it's been going up and seems to have stabilized, but it's still all over the map depending on the specific property and who is doing the insurance.

Similar to you, I'm thinking about other strategies for off-market leads and tried direct mailers, but not sure how effective that is for multifamily.

I don't know what market your rental is in.