I agree with everything rScotty and Fallon says and would just add to check all around and under for unusual damage or abuse.
Check the radiator and hydraulic oil cooler to see if it was regularly maintained/clean. Keep in mind the
M59 hydraulics generate a lot of heat. And watch for the usual rodents nests, use of OEM filters, dates/hours written on the filters, condition of oils, slop in pins and bushings, broken/missing bolts+zerks, evidence of regular greasing, bucket cutting edges, bent stuff like buckets+hydraulic lines, loose wires, operating gauges and panel lights, paint peeling from stress cracks, ANYTHING that looks like it was welded by the owner(s), electrical taped hand twisted connections, anything with duct tape, blah blah blah.
Don't forget to make sure the 3PH equipment comes with the tractor these can be expensive to add on later.
I would look for a one owner unit and avoid rentals and machines that look like they were immersed in water (i.e. watch for mud in odd places like electrical fuse panels and connectors)
I rarely have to use 4WD because the machine has lots of weight and push, especially with the backhoe attached. I'd be surprised to see a machine with 4WD always engaged. 4WD and locking diff should shift/work smoothly and without excessive noise in both directions and driving serpentine. Raise the front wheels off the ground and in 2WD - you should be able to rotate the tires and turn the steering between stops freely and feel no skips or odd noises in the front diff.
Condition of rubber tires. Drive the machine in every gear in hi/low range.
Have the owner demonstrate complete backhoe removal, see that all the PTO and 3PH functions work properly. It should come off smoothly and without tools, hammers etc. Same with the front loader QA bucket.
I'm always suspicious of going to see a machine and finding the seller has it already running and/or idling when you arrive. I like to see where the owner stores their equipment and how well their property and other equipment is maintained. I avoid equipment that gets stored outside unprotected or under a tree somewhere. Remember that ALL rubber items and seals have a useful life and they degrade exponentially when left out in the elements.
In general I'd say that the Kubota's either run forever if cared for or are broken because of abuse. Run away from the latter ones.