Just received a call yesterday that the washer is now broken ( I'm thinking that with an extra 3-4 people using it, of course it would break faster.)I called to get an exterminator out there at a cost of $900.

When we returned we did some significant upgrades (windows, roof, privacy fence, tile flooring) leaving the house attractive to potential buyers and we have all but kissed that money good bye that we put into it.  

Also- I don't know what manufacture you are looking at- but you can always do standard construction and upgrade the exposed cabinet sides to a real wood veneer.

If you know about it and don't disclose it, you've broken the law. 

I need to replace an A/C unit, fix a heater, put anti-tip on all of the stoves, fix some GFCI and some windows are broken.

The level of upgrades needed for the rehab has to do with pricing the resale into the market  fast and at a price that will be quickly acceptable and hopefully purchased with multiple offers to bid the price even higher.  

They say that dust and debris from grinding out the joints also got in/on the AC unit and some fins were bent.The tenants ask me why the guy didn't cover the AC unit or ask them to remove it from the window.They did not say that it's broken, so I assume it's working.

Would appreciate feedback regarding smart owner upgrades/renovations to make, or avoid, for 6 unit middle income apt.

Make sure any upgrades you do are very durable and only things that will save rather than cost you money in the long run, are typical for low income properties, and are thing slow income applicants value.  

Some of the people you'll want to make good connections with are Realtors, Lenders, Closing Attorney or Title Company, an appraiser, a lawyer, and possibly a trustworthy contractor or handyman if you'll be making repairs or major upgrades.