Speaking from experience I try to get deals where I can... say rent is 600 a month retail hvac unit duct work is 5k ok after taxes insurance that means the tenant has paid a year long not missed a payment your still in the red... that’s why I pay cash.

But they are also ugly, noisy, and expensive to run, so it depends a lot on the class of property and what your goals for the property are in terms of repositioning.It isn't always as simple as just slapping in a central unit; Electrical panel and/or service upgrades are sometimes needed, and you need attic or crawl access for duct work and returns, as well as a place for the air handler to go (which often means giving up a closet). 

Running HVAC trunk lines to the old ductwork was another fun issue.I pulled the permits, had the inspections (prior to & after drywall) & passed.

All of the duct work etc has been ran. 

Looking in the crawl space the duct work seems to be fairly new and the plumbing looks good from a quick look under the sinks. 

Most have added room additions during some point in time, so ductwork can not be continuous on any floor. 

I'm also finishing the 2nd floor/attic and will need to put in ductwork.

Of course, to replace means you probably need to install new furnaces and duct work in addition to all of the patching and soffit work that will require. 

Compare that against the cost of having two furnace and air units installed and all Freon lines and duct work ran through the ceilings and floors.If a unit ever goes down in a cold winter or hot summer the tenant has a second unit as a back up to use until the broken one is repaired.You will not have to get the call saying a heroine addict has stolen an entire outside air unit for scrap and you need to replace it asap because its 94 degrees outside.No pilot lights to go out on a furnace. 

I also believed that HVAC had to be ran, but the wholesaler said the duct work was already there.