I don't know a lot about McCormick tractors but know that they are pretty much rebranded Landini tractors painted red. With that said Landini seems to be a pretty popular european brand and the McCormicks I also believe are assembled in Georgia.
Things to ask.
1. Is there an authorized dealer nearby. Although most dealerships will work on any brand of tractor an authorized dealer can usually get parts cheaper and will be aware of any manufacturer TSBs. (tech svc. bulletin)
2. At 18 hours it hasn't even reached it's 50 hr svc yet. Usually for hydros a 50 hour svc on the trans is typical with a filter and fluid change. It pretty typical to have shaving of metal as the gears help seat into place. So if this is a dealer will they change the fluid/filter prior to purchase or at the 50 hr svc.
At the price you got listed with the amount of hours it has is seems to be a pretty good deal especially for a hydro. I've been looking at McCormicks lately. I have seen a 50hp, cab, 4wd hydro listed by a local dealer for 23 grand and although I have a kubota and would like a deere my pocketbook keeps steering me toward McCormick.