Certainly pull an oil sample. Also check the front cover well for cracks, you can do that when you check the bull gear. It should be a pre-egr, so I would have about $16,000 set aside for an inframe. That's a little over 2 cpm if you get another 750,000 miles out of it, and around 1.5 cpm if you get what it has now. Plus it's an opportunity to rebuild it with fuel saving upgrades. In addition, once you have it rebuilt, there is a peace of mind knowing exactly how it was done. It is a cost efficient motor that can make you a lot of money. I wouldn't hesitate to buy it, but this would contribute to how much I'd pay for it.
My experience with my Detroits;
Twice I've done the 1999 series 60 at 762,000 and 785,000 doing oil samples, running synthetic & had Gulf Coast by-pass systems on them. Changed them over to synthetic at about 600,000 miles when I bought them. I have two friends that have over 1,200,000 miles and never have had to do the bull gear or in-frame. One is a 98, the other a 99. The one at 762,000 went when I was driving it. I heard a clanking sound, immediately turned it off and coasted to the side of the road. When my mechanic got into it, the piston broke in two. The sound I heard was the two halves of the piston hitting each other. The other time a driver was on it, it shut down, was low on coolant and couldn't keep coolant in it. We were able to drive it to the shop though. Replaced the front cover at an additonal cost of $1,000 on the one that broke the piston because it was cracked. He said he could "patch" it and it would be fine. I elected to buy a new one as that's how I prefer to take care of my trucks. They may be old, but if it breaks, or may brake soon, I have it replaced. Plus, I always pay the trucks first, then myself, so the money is always there to fix them.