As Rhill said it depends on what year you are looking into.
The CJ's made by AMC (prior 1987) were notorious for rust as were the the latter YJ's and early TJ's made by Chrysler. However the later model TJ's (up to 2006) had significant improvements made in rust preventative as it was always a major complaint about Wranglers. As for the JK's, I'm not a big fan so can't really comment. The Wrangler legacy ended in 2006 for me! LOL
(I know, I'll catch hell for that comment!)
I guess it all depends how the previous owner took care of the vehicle. I've lived in New England all my life and have had CJ's, YJ's and TJ's and never really had an issue with rust on any of my vehicles.
I was always vigilant in Frequent power washing the under side of the jeep, especially in the winter from road salt, and after running the beach in the summer. (Before Race Point beach permit prices got ridiculous - and the "Plovers" took over!)
Best advice I can give: As with any new/used vehicle purchase if your not mechanically inclined, you should have the vehicle checked thoroughly by a mechanic YOU trust before laying down a penny.