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.

I have had a couple of chairs broken, but they were garage sale chairs anyway that were bound to go at some point.

Also did you start on BP as basic and then upgraded??

I moved in and immediately started with upgrades on the place, mainly a kitchen renovation that it needed badly.  

House hacking (buying and living in the unit while making repairs/upgrades, rinse and repeat) may be a good option as it can be hard to find properties that cashflow in this area but making upgrades to distressed properties can build you equity right away.