To upgrade A/C unit and furnace or not?

[Joel Phillips]

beginning my first rehab house (about 2000 sq ft house, 2 story on a slab)

I walked through the inspection and took tons of notes. I have my list of projects and a budget. My question is this:

The A/C and Furnace are original (1992), and both seem to be working properly when we tested them.

Is it worth it to replace them? do you get a return on investment? Options include replace with new equipment, or replace with used but good condition equipment, or leave it.

thoughts?