If it's on the duct work typically people just encapsulate it so that it can't be disturbed and it's the most cost-effective and easy method.

No ductwork needed, and it supplies heat and AC.  

Central air/heat duct-work.

Extra costs for insulation are offset by reduced costs for HVAC equipment and ductwork. 

(Note the furnace is central heating so no new ductwork should be required).What do you think renters would most notice and value?

Is there ductwork already in the building?

We have done several homes 1300-1500 soft with a completely new furnace & central a/c for $5400-$7000 depending on the extra duct work etc needed for new runs. 

It wasn't just a little rehab - put in CHA unit and all ductwork (it had window units), tore out the 4th bedroom to make it back into a carport (more desirable than extra bedroom), ripped off the walls on the enclosed porch to make it a screened porch again, all new energy star custom windows and doors (got them on a holiday sale at Home Depot so 15 % off and had a 10% off Lowe's coupon that I had HD honor and got 18 months 0% interest) and painted inside and out (Pro Plan paint at HD - another discount and same 18 months financing), new appliances (scratch and dent with 10% off and 18 mo. financing at Lowes), and new countertop and ceiling fans.  

@Lee Gierszewski - on my recent flip to rent property, there was gas forced air with ductwork in place.  

Opting for multi-zone mini-split systems (link is just a sample, there are many to choose from) offers a practical and efficient solution for heating and cooling an entire house, particularly in renovation projects where space for ductwork is constrained or entirely absent.