I live in a climate where every 7-8 year old vehicle looks like that or worse.
IF I were to buy it, I'd strip a few things like the gas tank skid plate, bumpers, tape up sensitive things like the wiring/plugs, put it up high on jack stands and have someone come and glass-bead blast the entire underside. Then, having taped off around the bottom edges of the exterior and covered off/wrapped anything you don't want covered in paint in tinfoil or tape, then I'd shoot the entire underside with epoxy primer from a gun. After that, top-coat it with thinned Rustoleum or another top-coat of your choice. Then I'd wash out the insides of the frame rails with a pressure washer, allow it to dry, then apply something like Eastwood's internal frame coating which is zinc-rich and acts as a bit of a sacrificial anode, and is also said to encapsulate any existing rust.
At that point, having cleaned up and sealed the frame and underside as best as possible, I'd shoot everything including the insides of the frame rails with a creeping oil-based product like red Rust-Check, and a few months later I'd hit everything again with something thicker like Fluid Film. Then your existing rust was all removed/encapsulated (as much as possible) your paint coating was restored (as much as possible), and a self-healing, creeping anti-corrosive oil coating should take care of the rest and slow down further spread.
Of course, it'll never be as clean as factory and this doesn't solve the issue of breaking off already-corroded bolts when you try to remove them. I've worked on some rusty stuff and you can bring it around, especially like this one where it looks mostly superficial and hasn't become "rot" yet. But, it's not nearly as nice as crawling under my rust-free 80 and gently cracking a bolt loose with a ratchet and threading it out with my fingers.
If it's significantly discounted vs a rust-free one and you're up for the extra work, I'd say this one doesn't look bad.