Around my area it could also mean it was purchased prior to the new system they installed in 2000-2001 when the data didn't exist in the import. 

I have found the deals, redesigned the house, put together the budget, and did the rehab.

They are fairly inexpensive (about $300 each installed by a contractor) and can make a huge difference in your house.

Currently has an ugly commercial drop ceiling installed - how we decide to tackle that one will depend on what it looks like underneath.

I would try to aim for outswing units, as they tend to have less leaking problems at the sill.The main thing as far as install goes is you need to make sure to set the unit plumb, square and level, otherwise you will always have issues.

If it gets hot upstairs install a window AC to cool it before showings but other then that leave it alone. 

However you can install a couple of large footings at the end of each wall and install a beam, maybe a steel I bean into the ceiling and to which the cross ceiling joists connect using joist hangers.

On my latest rental, the water heater is original, installed in 1997 (contractor's grade, nothing special).

My broker does mid-century modern rehabs, new construction and consulting in Palm Springs and San Diego (focused mainly on the Point Loma area) and we find that having top quality installs is key but basing the cost of the project on what is selling in the area.

Rather than install new ones, you could save a bundle if you are willing to put in the work to paint.