At the recommendations in my owners manual - at 50 hours I changed EVERY fluid except the diesel fuel. Front axle housing, engine, hydraulic plus the two hydraulic filters, engine oil filter and two air filters. I do grease all 36 zerks on the tractor, rear blade and grapple, at least, every ten hours.
I guess if you are very concerned and the current owner agrees - you could send in samples of the engine & hydraulic oils for analysis.
Otherwise - take a leap of faith - buy the tractor and change the hydraulic fluid/filter(s), engine oil/filter, fuel filter, air filter(s) and grease it.
My initial concern is not the hours on the tractor so much as the ten years with the same hydraulic oil.
By the way - where are you and this tractor located - location will have a bearing on this situation.
And if you are saying that the ten year old tractor has only been greased once recently in ten years - that would concern me also.