+1 on chase down the moisture and fix that problem. Could be any number of issues causing that. Bad windshield gasket is common, leaking heater core is common, even door seals. Until you fix that problem, it doesn't matter what you put down, everything will hold the moisture against the floor and rust will follow soon.
Once it's fixed, just plain old carpet underlayment that most of the auto carpet sources sell. Killmat or Dynamat are the two sound deadeners that seem preferred by car audio installers if you're going that route. The sky is the limit, but it all depends on what you want to use the van for in the end and a lot of personal preferences.Carpet Link