Most made 4-5K in 1480's and the few 1680's i know of. 2 of the 1480's we had had the smaller 436's in them and they made near 5K.
If it has been overheated or poor maintenance it won't matter what engine is in it. Check it over, run it some and decide then, Take along a mechanic that knows those engine if you are not sure yourself.
I remember back in the 70`s my Dad had a 466 in a 1066, and a 4366 and 4386 and they worked well but then they were new then. After looking at 1680`s it seems that 75 percent of them have had a engine rebuild before 3000 hours. This one I am going to look at is at Richie`s sale so I have no idea where it came from or the history.
They're good engines. I believe the biggest problem is that they have a problem of spinning the rear seal. I just had to have mine replaced along with the oil pump. If you ever have to replace the seal replace the flex plate too
Ya, check blow by, Check for moisture/antifreeze on dipstick, Make sure the guy doesn't start it for a day or so before you look at it. With decent batteries it should start at 5-10 below C with out either or plugged in.
Also outright ask the guy how it runs, if he has done the pump/injectors and or turbo? The combination of hours and age I would do all three as well as the lift pump down at fuel tank if he hasn't .
Check the condition and how clean the rad is . Check coolant level and look for oil scum on rad cap.
If you have time pull an oil sample after it has ran for a while and send it away.
We have a 466 in the one 1480 we still have (don't use anymore) and 2 in 4366/86 tractors. Except for oil leaks on the 4386 they have been great engines, excellent cold starters.