Like Dave pointed out... bio/geo/email info will be helpfull.. however.. here are some yanmar tips in general.
Lots of places have claimed to import factor refurbed yanmars.
It is up to you to determine IF they really were refurbed, or just pressure washed and painted.
Refurb is a pretty vauge word... it might mean that they were bought and that major defects were corrected, and restored to a running state... i.e. fluids/filters changed, tires repaired/changed... obvious broken parts and non working parts replaced.. painted and out the door.
Then again.. you might actually have a dealer that rebuilt this engine.. anything from just an upper or lower rebuild.. to the full engine job.. possible even hyds, and front axle issues.
Look at the tractor... you can tell what a cobbled up repair looks like.. in most cases, you can even spot new repairs.. look for gaskets that arent painted like the rest of the casting.. or even the telltale rtv silicone gasket material extruded between two castings.. etc.
Look for leaks... check oil to see if it is low, dark, diluted,or emulsified with water... Same with hyd level.... check the function of the tractos hyds... ask to test drive.... listen/feel for engine transmission smoothness in all gears / ranges /throttle positions. .. just general engine stuff... look for excessive black smoke or slobber from the stack ( all diesels wll occasionally emit black smoke.. etc.. just the nature of the beast.. ).
Look at the electricals.. like gauges.. and charging system.
See if there is side to side play in any of the wheel bearings, by trying to rock the wheel side to side... good place to look for axle leaks is at the hubs, and on the insides of the rims.. etc.
Look for leaks around the pto, and front axle seals if it is a 4wd.
Find a dealer you can trust... you will need that dealer for parts and service.. so make sure you feel good about him, and the tractor.
Keep in mind it is a used machine. If it was well maintained, or rebuilt well, it has thousands of hours of life left. If it was abused.. or reapired just enough to limp off the trailer.. then you should be able to see signs of this when you inspect it.
This is all pretty much general tractor stuff.. not just limited to yanmar...
Good luck!
Soundguy