You didn't mention the year of the Jeep or what type of rust (read: surface, flaky?). Is it rusted through in spots on the frame? Just because you see rust doesn't make it a bad buy. All Jeeps eventually get rust underneath.
My '97 I purchased had plenty, but everything was sound. Spent two days underneath with a small hammer and wire brush knocking it all loose and cleaning it up.
Once you get that done you can inhibit rust from coming back with Rustoleum, or better yet, a product called "Chassis Saver," though the latter is easier for coating the undercarriage using a sprayer.
Previous poster did make one good point... why would you buy a vehicle without looking underneath? Basic purchasing. I hope you at least kicked the tires.
I live in the mountains of CO. Have to deal with Mag Chloride, it makes everything rust. You can wire brush most off and use Chassie Sealer, its a black paint like material that is put on the metal and seals it forever. I've seen it on AutoGeek.Com, good luck