In Huntsville there is no reason to buy a jeep with rust anywhere. You can bet that the diamond plate is covering rust. You can also bet theres going to be more rust out of sight than visible. The mileage is unknown. For 6k you should expect a unit that needs only some cosmetic work.
If you are good with tools and welding equipment, I would offer no more than $3k. If you are not inclined to mechanics I suggest you walk away from this one.
As an aside, when I was looking to buy Craigslist had a CJ7 for $6k I went to look at. It had minor rust, but the major thing was that it would not shift into gear because of a bad clutch. The owner who claimed that "it was working perfectly yesterday", told me that "this is a collectors item and cannot be expected to drive daily". I told him until he fixed the clutch so that I could at least drive it, I could not be expected to even make an offer. I asked him to call me when the clutch was fixed, but he never did.
Beware of "collectors items". They are just vehicles and there is no shortage of them out there. Don't settle.