: Current toilet is a Standard Cadet mfg 1939. 4" cast iron drain. Well water. Plenty of condensation during humid summers.
: I like the idea of replacing the toilet with a lower volume flush and getting rid of the condensation.

: My concerns are:
: Clearing - Will there be sufficient wetting of the 4" drain to adequately clear the waste? What models would you suggest?

Many will work with the 4".
Of course the better ones will have either the 3" flush valves or pressure assist.
The Toto models allow more water to be used if the handle is held down longer.

: Condensation - Will the lower volume be enough to remove the condensation? Or should I look at tank in a tank type models like the Vacuity? Perhaps a Toto Drake insulated? Other models?

Since toilets now use 1.6 gallons, "sweating" has been less of an issue. Most of the water stays in the tank, so the water does not get as cold unless you flush it several times quickly.
"Insulated" or "tank in tank" will help too.
At this point, I would not consider the Champion with one of their leaking tanks. It would be a big "sweater" for you.

: Rough In - Mine is 18". Does this limit my choices? Should I have it changed to use a 12" model?

Wow! Yes, you would pick up another 6" of space in the bathroom.