Thinking of purchase for son. Is this too far gone, or fixable?

Its not so bad IMO. If you are really concern with rust on these frames, there are avenues to pursue. https://www.autorust.com/product-category/toyota/4-runner-1996-2002-3rd-gen/ On my 3rd gen i had to replace the rear frame mounts area. As long as you have a reputable mechanic or welder its not a hard job.

All in all, i don't think its that bad. Always keep up with routine frame maintenance as far as using rust inhibitor products. POR-15, fluid film and rust oleum products and there are others that names escape me, but same idea.

GET IT!!

 

Best I can see from the photos is that the frame looks to be OK for the most part, the worst rust appears to be in the first photo, the very rusty piece shown is the gas tank skid plate which can be easily replaced with one from a savage yard or aftermarket part source, there could be other rust concerns but not shown in the photos you posted.

I would think you could have some bolts that may break when trying to remove them to do any work on the vehicle, but that somewhat normal when you see any rust under a vehicle.

If you do buy it I would have the under carriage/frame cleaned of rust and then treated with a rust preventative to stop any future rust.

 

It depends on the asking price. $2k-$3k maybe, but $6k or more walk away.