Back to the original question:
From your description, the baler has been well-kept, the owner has manuals and a few spare parts, all of which are positives. For me, after viewing the general appearance of the baler, I would focus on the belts. Frayed edges, bad splices, cracked / dry rotted rubber, belts that appear to be stretched or on the other hand, much tighter than the others, and multiple pieces spliced into a belt are all indicative of belts past their useful life or it could mean you need to take them off and re-cut and splice for a even length. A new belt will cost you $250-300, maybe more with current inflation.
Other than that, usual mechanical stuff. Have him hook it up and run it, operate the tail gate, etc. Look at chains, sprockets, hydraulic hoses, hydraulic cylinders, and so on.
I think New Holland recommends 60 HP for these balers. If you roll a smaller bale, say a 4x4, you might be able to get by with slightly less. You will be happier with at least 75 HP though.