Check with Bush Hog or a dealer to see if parts are still available for it. If you buy and the gear box grenades 6 months later, can you get parts to rebuild it? Or, even little things like blade bolts, blades, slip clutch discs, etc.
I tend to buy more recent vintage implements because I have very little free time and want to spend it working WITH my equipment, not ON it. And I don't have time to spend hours searching for parts no longer commonly available.
Now, having said that, I bought a Gannon box blade that was probably on the Ark with Noah, but there's not much to go wrong on a box blade.
As for things to look at when you see it in person:
1. Grab the input shaft to the gear box and see if there's any play in it. Then turn it to ensure that the gear box turns smoothly and doesn't make any grinding noises.
2. Look at the underside of the deck and see what it looks like. Are the blades and/or blade carrier (stump jumper / blade bar) beat up? Also, if you can see where the gearbox output shaft comes out under the deck, see if there's any oil or grime buildup, which could be a sign of a leaky seal.
3. If possible, run the cutter and see how it operates.
4. Check the gearbox oil to see if it's milky or appears to have any water in it. Smell it to see if it smells burnt.
5. See if the PTO shaft is in good shape and slides in and out smoothly. Half of the shaft doesn't have a shield on it and will need to be replaced. Also, the gearbox doesn't have a shield on it, but you can make one or have one made easily enough.
6. Look for any bends, welds or other signs of damage or abuse. Some of the side skirting looks like it's bent.
7. Check the tailwheel to make sure it turns and rotates like it should and see if the wheel bearing appears to be in good shape.
8. Look carefully at the slip clutch. You really can't tell much without adjusting it and operating it. But look to see if it appears to be rusted up or painted over (which could indicate that it's not slipping like it's supposed to). Or the slip clutch discs could be worn out, which allows the slip clutch to slip too much (and then they need to be replaced).