But we have also added central air to house/apartments where the heat is hot water and have installed all new ductwork. 

I recently had a HVAC unit installed running duct work and that alone was about $18,000.

If you have shared ductwork, you are wasting your time and money doing sound proofing.

Each unit needed all new plumbing, electrical, duct work and HVAC, 4 new bathrooms, 1 new kitchen, paint, finishes, flooring, etc.

I will say the majority of the time HVAC does not produce significant ROI unless you already have a forced air system and you can tie the AC portion into the existing duct work. 

Off-market legal 3 unit in the Bridgeport neighborhood of Chicago, comprising of a brick 2 flat with (2) 3 bed 1 bath units, and rear frame coach house with 3 bed 2 bath.Each unit needed all new plumbing, electrical, duct work and HVAC, 4 new bathrooms, 1 new kitchen, paint, finishes, flooring, etc.

Out here in Phoenix/Scottsdale you're only putting a mini split into an addition that you can't run duct work to, a detached shed/guest house, or a garage.

I also have some minor ductwork that needs to be done.Any contractors you can recommend are appreciated.  

I would guess he does both for <$5k if you already have ductwork ran.Luis-773-575-3423(Let him know you know me!)

The previous owner had added central air for the upper floor, installed in the attic with all ductwork in the attic and defusers in the cieling of the upper floor.