Do they have any maintenance records on that truck? Was this their own truck that they used, if so they should have maintenance records.
Not to sour you on this at all but if it's their truck and they're not showing maintenance records and they don't have records of when the engine was rebuilt I myself would see that as a problem. All trucking companies keep track of the repairs; it's required that you do that. Plus you need to deduct all that from your taxes.
Here's a couple things to check. Look at the cab mounts and look at the motor mounts. Trucks that have that kind of time on them generally need them replaced. And many many owners will do other things but put that stuff off. If that stuff has been replaced that's probably a good sign.
It's hard to tell with the fifth wheel on but try to get up under there and look and see if the frame rail is still thick. Sometimes where that fifth-wheel sits the frame actually rots away. Check the wear on the fifth wheel.
I would also take a good look at the cab air bags and shocks, I would look at the springs on the front end and the shocks and I would look at the airbags, shocks, and bushings and slack adjusters and air cans and if you can get underneath and check the S cams.
If none of that looks like it's been recently replaced it can be a lot of money when you start going over the truck. This is why having the maintenance files are so important because you can see that two years ago they replaced all the slack adjusters or a year ago they did the air cans on the rear axle.
Also look to see if things like belts and hoses have been replaced or if any of the fuel lines under the truck have been replaced and if it has the off on valves for the fuel tanks check them and see if they work. I changed mine into something different but I think they're about $400 a piece from Freightliner if that truck has that option on it.
If they use the truck off road I would thoroughly check that front end or have someone check it because you may need Springs and hangers and bushings. Make sure you check the kingpins and the steering box too.
I'm not suggesting at all you don't know what to look for but if it were me I would get a flashlight and crawl around and try to make sure I don't see any leaks at any wheel SEALS or Driveline SEALS or any of that. I would get underneath and I would shake each pinion joint to see if there's any play in the pinion or the joints. Same with the back of the transmission and the seal.
You can see why having maintenance files is helpful.
If you buy the truck I wish you good luck.
If you don't then I hope you find another fld that has maintenance files.
Dollar-for-dollar I really do think they're the best trucks out there.
But before you start running it you need to go over it from front to back and try to Bulletproof it the best way you can.
Whatever you put out up front, dependability is the name of the game.