3000 hours on a CUT (with their 1 to 2 liter (60-120 cubic inch) engines might be ready for an overhaul. Those same hours on a full sized utility tractor might only be 20-40 percent of it's usable life (before an overhaul is required).
Lower hours would be better, of course, but a "rule of thumb" would be pretty much guess work. Even lower hours with a machine that hasn't been maintained might not be as good as a higher hour machine with proper maintenance.
So...you need to be more specific with what you're looking for.
Roy is right....But I think it TOTALLY Depends on where/who the machine is from. I wouldnt make a blanket statement saying that 3k hours = an overhaul...
I got 3600 hours on the 3TN72 in my F935 and she starts & run just like a new one. BUT, it came off a golf course which took good mechanical care of its equipment. Oil changes came at set points, etc....
The way I see it is a "professional" or "for profit" outfit realizes that maintenance is not an expense as much as it is a key to profitability... Average homeowner John Doe might say "oh I only put a few hours on my tractor this year..no need for an oil change" etc... OR may be the type to buy the cheapest oil on the shelf because its "all the same"...
I've seen my share of machines which while they looked good, were far from mechanically sound...at the other end, I've seen some machines that looked like they were left for dead in the desert...but were mechanically in great condition.
I see "pre owned" machines in 2 scales...
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Beat Up...Beat up means be prepared for a project...and equates to cheap initial purchase price with alot of heartburn & work down the road. If uptime is critical, this is not for you.
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Used...Used machines look great, are in great shape, and typically sell for so much money, with incentives, financing and other considerations such as desireable options, its usually cheaper to go buy a new machine.
In my travels, I've seen very few machines which fall outside of those 2 categories in realistic terms. What I DO see all the time is people with beat up machines asking used machine price...
All of the above is really dependent on location...the tractor market both used & new is fairly strong here in New England...