I'm buying a New Holland with 31hours on it in 6 months of ownership - I think that's low.
You may get an idea from the manufacturer's warranties:
Kubota is 2 year/1500 hours - 3 year/2000 hours powertrain on the L series. My NH had a 5 year/3000 hour warranty on the emissions system. That being the case, it sounds like manufacturers expect 600-700 hours per year on a compact tractor.
I would want to see maintenace records for any used tractor I bought, plus it would be worth it to me to have diagnostics done on it from a trained mechanic. I understand that the warranty on a tractor follows the tractor (not just the original owner) BUT you may not be a priority unless the tractor was sold from that dealership. Kubota's own warranty states that "dealer's own customers may have priority."
New is great - no warts and a full warranty. Used within a warranty period is great - less depreciation and partial warranty. Used is generally "as-is" - unless you negotiate a warranty from the a dealer's used lot.
Good luck!
Mark
PS There are a couple bargains to be had out there new - quality tractors that are priced like used ones. I would specifically steer you to looking at Branson, Century, TYM/Scorpion, Mahindra (the KOREAN ones!), Long Agribusiness (again, the KOREAN ones!) and Kioti IF (big IF) they can be had from a reputable dealer with a good service shop.
PPS I didnt know that Deere measured hours at PTO speed. All the hour meters that I have installed had a sending unit off the ignition, so the clock ran when it sensed electricity in the systerm regardless of what the PTO was doing. Makes sense if it is mechanical - slower engine RPMs = slower spinning of the clock.