If you currently have a forced air furnace in the duplex, an a/c unit may not be too expensive, and probably won't require any additional duct work, assuming the furnace is relatively new and a compatible a/c unit is available. 

We have a quote of $385 to have this replaced with proper duct work."

If I could add to what John Leavelle said  ; it could cost as much as $60,000 to do a full Rehab here in the Philadelphia Area        My Friend Sean just did a Rehab in the Port Richmond Area Close to Fishtown and he it was $60,000        Sean has done many flips and I thought he went as "Budget" as you possibly can , for an example ; he paid $7,000 for a brand new HVAC  (Heat & Air Conditioning)  with new duct work , where that would have been double in the open retail market       Good luck with the project & feel free to reach out to me if you need any help or have questions SincerelyRick      

So don't overlook that you could get some of the high velocity stuff for some areas if you already have duct work in limited spaces.

Unless it is an older home (ie radiant heat and no duct work) I would add it if at all possible. 

For AC, you need separate ductwork for supplies and returns between the units, separate blowers, and you also need separate cooling systems - condenser and coiling coil; you might be able to use mini-split units to achieve that.For hot water, you need the piping separate for the hot water, and then you will need multiple water heat sources with one per unit.

No additional duct work needed, able to use in houses like Eichler, flat roof homes.

There are some skills that I suggest every homeowner learns in order to avoid high repair bills (in order I usually do them):-Fix minor roof leaks & repair siding/exterior-Fix water supply leaks with compression fittings or hose clamps if needed-Install lights, replace bad receptacles, switches, or breakers-Install electric water heater if needed-Fix subfloor & sturdy up framing if needed-Fix/replace windows/doors/glass and install locks-Drywall/mud, brush&roll paint-Install floor (floating/tile) and cut/install trim-Install toilets/sinks/cabinets/faucets-install mini split units if no functioning ductwork

In July I completed a lot of updates to the house (new well, new 200 amp electrical service and panel, new windows, replaced forced air ductwork, added a bathroom. 

there is nothing wrong with it, most microwaves have that option and do not have to be vented out, even though as a contractor i prefer them to vent out. if the existing pipe already vents out, which it probably does if there is a pipe in the cabinet already. what you would have to do is unscrew the 2 or 3 screws in the top cabinet that screw down into the microwave ( if not there then there may be 2 bolts in the front of the unit behind the cover that have to be loosened) that will loosen the microwave, then its a matter of taking the screws out that hold the fan at the top back, pull the blower fan out and change the direction so that it faces up, screw the fan back to the unit and put the microwave back up and bolting it. check to make sure the fan connects to the duct work, if not, add a piece of sheet metal to connect and seal that joint. btw you may want to unplug the microwave before doing so, just for safety reasons.