As an owner of a 1984 1710, yes parts can be hard to get unless you look to a tractor salvage yard. All tractor OE parts are going to be expensive, no matter what brand, model, or year.
That unit has probably spent it's life in the open, hence the rust. The bucket probably was kept in a carrying position so water would have set in it. You could get a salvage bucket for it so that should not be a big issue.
Most of the tractor should be fine as long as the hydraulics work well, it shifts well, and there are no abnormal sounds when running it around the yard in the various gears. This tractor should be able to run out for over 3500 hrs. With this age and exposure the hoses and tires have taken a beating. While the exterior of the hoses are cracked they still will hold pressure. I would probably replace the hoses which can be done from a on-line source noted on this forum for not too much expense. Three of my tires are original and suffer from there age and abuse (tree stumps), but have good thread depth and hold air well.
Other then the sheetmetal, the rust should be surface (bucket exception) and just going to take a fair amount of work to wire brush / grind off, then cover with something like POR15. Normal painting can go over this to make it look pretty again.
I would probably offer 4000 to 4500 for it considering the bucket, hose condition and rust. If the owner knows it's solid underneath, he might stubbornly want a few hundred more.
