Ask for any kind of proof of maintenance, filter change intervals, replaced parts, hours on the machine and who does the maint. (self or factory). Check the blades to see what the machine may have had through it. Dull is one thing but chips and chunks out of the blades means perhaps things other than wood has gone through. Check the bearings on the drum making sure it's not wobbling. Visually check the oil for water contamination. Check all the hydraulic lines for wear or leaks. Check all road lights for legal travel. Run it and have the owner demo the unit too running the max sized logs through the unit.
Good luck