The vehicle in the video is "honest" - you can see the metal, mud and rust. It needs to be protects ASAP with a lot of Waxoil inside and out (or something similar). The benefit of waxoil and other wax / spray type things is that they protect but don't hide a lot. The problem comes when the whole thing is sprayed with a thick undercoat. It will look great but there could be very little metal underneath, and chassis corrode from inside. The edges of the holes in that chassis look ok, you can see the thickness, there are some damp patches, OK if they are oil, but bad if they are wet leaking from water trapped inside. Can't tell from the video.
"done" - rust treated inside and out and welded where it needs to be or, sprayed with thick underseal over paper thin metal. both look the same and the only way to tell is poking hard (if you know where but its still a gamble) or the seller has before and after vidoes of the work - but if its a dog who would vidoe it? That's why its often better to buy scruffy but "honest" vehicles.
When I bought my S2A I spent ages under it and the guy clearly didn't think I would buy it as there was chassis rust. But it was localised and repairaible, nothing was being hidden, and I felt I knew what I was letting myself in for. Welding over the next year cost around £800 but it was for sale around £1200 under the market rate.
