Duct cleaning would be an absolute must if you had major mold problems or some other health hazard, but the coil cleaning and ozone alone may get rid of the smell in the ductwork if you leave the HVAC recirculate fan on while ozone running, in which case you could leave duct cleaning up to the next people. 

Here in South, you pretty much need central AC anyway these days, so once the ductwork is there, switching to central heat is a no brainer.Edit:  just noticed that you ARE in NC - I would replace them then, unless you are in a D area where window AC units are feasible

I was also able to raise the ceiling by 14” because I switched to exposed spiral ductwork.

Central air systems in these homes would have been added later on, so ductwork is often located in a very small attic crawl space.

It will go everywhere air will go - including inside closets, hard to reach spots, inside the duct work etc. 

Another trick I have done in the past with homes with the old high ceilings was to run ductwork up from the basement and strap the ducts to the ceiling.

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.

Or is there an inspector or someone that can check the duct work or air leakage?