Ripper's got it mostly covered, though it's probably gonna have some surface rust, but any major pitting is signs of trouble.
In my experience, cracking at corners of welds, like the corners of the door frame, are most likely to be the first signs of metal fatigue.
I'd probably shy away if it looks like it's been used with a bunch of firebricks missing, but they do get beat up and cracked from normal wear. Pretty much a maintenance item.
Being a furnace, check to see that the blowers work.
Door seals are another cheap maintenance item, not a big deal to replace. The door should open and close nice and square though. If the steel the hinges are welded to is warped, you may not get a good seal even with a new rope in there.
I have 0 knowledge on cat or afterburn setups, can't help with them.
Is chimney pipe or anything else included? Stove pipe, mainly the good double/triple wall insulated stuff is a big expense, and if it comes with, that's a bonus for you.