Ours was very abused old machine when we got it but it had the frame cracked just behind the fuel and hydraulic tank mounts on each side. We were able to weld and plate it and it has held up fine doing the things we use it for. Both of the ones I have seen have had cracks and repairs around the pivot pins for the boom.
Big thing is brakes and wheel seals on all 4 corners and CV joints on the back. I spent a lot of time this spring getting my 922b all dried up and brakes functioning on the front. Our 944 is missing the brake master cylinder/booster unit so it will continue without brakes for the foreseeable future.
If the machine does not have a cab make sure the center cover on the steering wheel hub is in place. The steering shaft is hollow and will allow water to get into the steering box and do all sorts of damage but there is one part that is particularly expensive in there.