Rust is easy to spot on a Mk3.
If the jacking points are mashed, most probably has rust you can pull back the plastic back quite a bit so take a lenser type torch and check well.
Walk away unless it is clearly only minor, that part of the car gets really sprayed in the wet and fills up inside making the rot spread.
The rest is easy to spot.
I have seen a few really solid low mile Mk3's of all engine types going for under £700