Good luck with the search. A couple things to look for: Common spots for rust would be edges of panels, along the rockers, inside of shock towers, inside of the fenders, around the trunk (particularly on a hatchback), and floor pans. (if it's got a sunroof, it can not only rust around the opening but if the drain holes get clogged it could lead to water pooling in the floor pans causing rust there) Take a look at the torque boxes that hold the front mount of the lower control arm. It's a weak sheet metal box that's only tack welded from the factory. Years of racing abuse or running on sticky tickers without re-enforcement can twist or crack them. Look for signs that the chassis has twisted, like body panel misalignment, if the visors tend to fall down it could be an indication the A pillars have twisted, and ripples at the back of the roof line, particularly on a coupe/notch. Like any car of course, always want to check for signs of bad collisions. Again, look at panel alignment and check along the fenders and radiator support in the engine for signs of bent metal or repair. When you get serious about buying one, I would recommend getting your own carfax report to review it. It's only $25 for one or $40 for three. It's cheap insurance and well worth it. You might be able to get a forum member to run it for you but even someone with the best intentions might overlook something you would catch having more invested in the sale. However, don't rely on the report as I've run them on cars I've owned and things as large as a roof crushed by a tree was not on the report. Last, the fox has a 5 digit odometer that rolls back to 0 after 99,999, so be ware of people claiming low original mileage. This is also a great reason to have a carfax report as it should list some history of the mileage and should indicate if the odometer rolled over. Don't want to pay for a car someone claims is say 35K original miles and find out later it's really 135K. Finally, never get too excited to cloud your common sense. If you spot a red flag, just walk away. There are plenty of others out there.
Hope that helps. Some of those were info passed down to me but unfortunately, some I learned the hard way over the years. Happy hunting!