If your HVAC equipment is +/- 20 years old and it's been giving you some problems, I would most certainly do complete replacement which includes; the AC condenser, the AC coil in plenum, the furnace / air handler unit, as well as update to modern thermostats. If the original ductwork was a quality installation, there is no need to replace any of it. Should also be able to use the existing electrical feeds to equipment.
This modern stuff just doesn't last as long as it used to. Too many lightweight aluminum and plastic parts, wiring and circuitry, and things are often installed in a less than desirable locations such as a dirty, damp basement, an attic, etc. Back in the day, most places had a furnace only and they were much heavier metal with a standing pilot light and not much to go wrong, of course they were inefficient too.