that is too cheap unless he is going broke/divorce,etc.
first things I would check are the planetary hubs for metal in the oil.
check the swivel joint for leaks,this in the belly of the beast and allows the machine to swivel 360 degrees,if its leaking at the top or the bottom big bucks to repair.
tracks,can run a grand per side depending on where you buy them and their quality.
is it a two speed machine? if so,does it work?? take it out if he will let you use it,put all of the weight of the machine on the bucket and raise the machine to one side,do the cylinders settle allowing the machine to come back down on its own??
do the same with the blade.
check the motor mounts,if they are the bonded type they are noted for coming apart and will let the engine move all over the place under load.
pull the radiator cap and look for bubbles in the coolant while the engine is running,any bubbles indicates a cracked cylinder head or gasket about ready to fail.
does the boom appear straight?? check all of the pivot pins for extreme looseness,especially where it attaches to the machine,if that pin is worn out and sloppy more dollars as its not an easy job doing it yourself.
run it at full throttle,hear any excessive hydraulic whine?? motor smoke at full power??
drive it,while driving,look for straight ahead tracking,if not,one wheel motor is weak and not keeping up.
try turning the machine,is one side faster than the other?? again,wheel motor or hydrostatic pump is shot,non of which are cheap.
look closely at the wiring,does it look to have been repaired/hacked/burnt???
how many hours are on it??? over three thousand and its almost due for overhaul.
flat out ask why he is selling it so cheap,ask if its been on its side,under water,etc.