It looks like a normal level of rust, but at the same time, a "normal" level of rust in some areas, can cause major problems.
At least it is a framed truck, so foremost you should examine the frame, and if the owner isn't too alarmed to let you inspect it, take a hammer and pick and pound and poke on the frame to see if any holes are made. If holes are made, no damage was really done because it wasn't doing anything useful in that area already.
It may need some replacement parts, now or soon, for example the leaf spring shackles can be a problem at that age, and brake lines, though I'm pretty sure I see a new brake line across the rear axle so possibly more than just that one has been replace already.
Then there are fuel lines, power steering, transmission, etc, all potential things to need replaced before long. Buying something this age is definitely a DIY project to get it fit for a few years of service if you don't want to have to dump a few thousands dollars into it from paying a premium for shop labor and shop parts costs.
I'm not even mentioning the things that any 20 year old vehicle might need like suspension, hubs/bearings, fuel pump, radiator, steering rack, or alternator, etc, soon, though some of these things may have also, already been replaced.
Also check the rocker panels and dog legs, a common rust area but this is more cosmetic unless you live in some area where the city or an HOA disallows rusty or otherwise mismatched vehicle body panels... and of course, that can be repaired too. Rusted out frame is the main deal killer, even a weak engine can be replaced before I'd do a full frame off restore on something like that.
So it is worth going to see, getting under there (better when not very bright outside then use a strong flashlight to inspect the undercarriage), and then if it passes your stage one inspection, get a mechanic to look at it on a lift. Could also have rotten bed or floor pan but sheet steel is easier to weld on than other areas of rust repair, especially if you can find preformed repair panels.
Don't forget to ask about maintenance and repair records.
You can clean and spray some, but it's going to be a dirty job, and remember that any repairs are far more of a PITA when fighting against rust but if that's all that is available in your area, otherwise you'd expect to have to ship a southern car up (or "fly down drive back" as someone already mentioned) to get much lower average rust, OR find some elderly person who didn't take their truck out often in recent years so it's in cherry condition. This might be your best bet but will have to jump on one fast when you find it.