Knob and tube looks like this, flexible armored cable looks like this, and conduit looks like this.If it's BX, and it needs to come out for some reason, about the only thing you can do is pull the whole wire and replace it with modern armored cable; you can't easily put new wires in the old metal armor. 

The only capital expense I can think of on the entire property group is upgrading/replacing the roofs on 7 of 8 units over the course of the next 10 years (Our plan is to put high quality, metal roofs on everything to save for expenses).

I also have metal screen/security doors on all units.

An alternative to central air may be some fan coil units.

I've had several HVAC units and coils stolen in the past, back doors and windows broken into even while the unit is occupied, and pipes along with wires stripped out and sold for scrap.

or just the condenser...or a coil and new furnace?...

I'd rather of a metal or commerical shingle roof instead of an asphalt shingle roof.

Like can I successfully take out the metal struts to make the window look better without ruining the entire window?

I drew a little dart-board on a piece of paper and taped that paper to the to my metal desk.

I wouldn't go crazy on the sink, even a single bowl sink will do fine for a rental, may even cost you less, even though that price looks pretty good. just a little FYI - the higher the number on the gauge of the metal, the thinner it is, that one is 18 gauge where one came up below it that was 16 gauge but over $200 - that one would be stronger.