Rust buckets are not defined by model, rather they are defined by where and how they spent their time since manufacture.
Few CJ5s or CJ7s survive the northeast/midwest salted winter roads unscathed. The steel body panels and frames were not treated with any rust preventative. Even YJ frames are found to rust severely in heavily salted areas. My nephew in Boston had a YJ that the frame just pretty much dissolved, no amount of patch/repair could save the otherwise decent Jeep.
When you find CJ5s, CJ7s, etc, that spent their time in the dry climates of Texas/Arizona/NM/NV/CA these rigs are often quite solid, showing only surface rust.
Just this week I've seen pics of a 50-year-old CJ6 from CA that was quite solid, only a rust patina on surfaces.
Price is also driven by geographical area. $3500 for a '77 in the Pacific Northwest may be a steal, while $3500 for a moderately rusted '77 in Pennsylvania may be too high for the market.
Pics of the rig and your location will help the membership here give you a better idea of the merits of the deal...
Line-X isn't a great idea for the frame. Ideally you would want the frame sandblasted, weld repair any cracks, repair/plate any rust-through, then finally POR-15 the entire frame. This seems to be the genreally accepted method for most builds.
Hope this helps...