It doesn't look that bad, the rear axle is the worst of it in them pics but still looks as though it's surface rust that will wire brush off, doesn't look like flaky out of shape lumps.
There doesn't seem to be much paint left in general, that panel with the 2 mesh pieces has no paint, but isn't rusted. You can see in the 1st pic the left main chasis rail, paint gone completely on some of it and paint peeling off the remained of it. Again, doesn't look rusted though. You can see bit of the right chasis rail in the 3rd pic with the spare wheel, to the right of the wheel, seems paintless and flaky paint but not rusted.
If you bought it you'd want to be cleaning the underneath and painting it now to preserve it.
I'm only going by the parts we can see in the pics though obviously. The sills aren't in them pics and if like my 90 series they seem to suffer the worst. But the sills aren't the most expensive to fix, probably the cheapest, as not much stripping to get to them. That's if they even need repairing, they could be fine and just need a clean and painting like them other places in the pics need.
