You'd need an air-handler somewhere downstairs, taking up the same amount of space (and nearly the same cost) as a furnace. I can only assume that the increase in efficiency is due to having the unit being within the building envelope, instead of on the roof.
Do a full, new split system downstairs. I might endeavor to have the old unit remain as a back-up, with its ducts removed and a simple grate on the ceiling. Although, at 23yo it's got to be on its last leg, too.
IME, rooftop units are for when you want more than ~5 tons of AC, and don't want dual-split systems.
Edited excerpt from pitzersonehour.com: (short answer: packaged are cheaper to install and cost more to run, split systems; the opposite)
What’s the Difference Between Split System and Packaged Air Conditioners?
Split air conditioning system
- Greater energy efficiency possibilities
- Split air conditioning systems have SEER ratings from 13 to 23.
- Labor costs more because you are installing a unit outside the house and also inside (usually in the attic or crawlspace, depending on the type of air handler). Also, the system can’t be charged with refrigerant until it has been set in place, which adds to the cost.
When you’d want this:
It’s a more cost efficient choice if your home already has a [reliable] furnace, and you are purchasing a new air conditioning unit or replacing an older unit.
Packaged air conditioning system
- Space efficiency - Unlike split-system units, the package unit has all the components in one place.
- Cheaper installation - Since packaged units can come pre-charged with refrigerant, and they are only installed outside, installation is cheaper than a split unit.
- Limited energy efficiency: Packaged units typically have a SEER rating from 10 to 18.
A company I work for will up-charge you if it's a horizontal attic install or in a crawl space ('cause that ain't fun and does require more work).