I would recommend a couple of things. For starters, see what you can find with a Carfax or VIN search.
Depending on what the VIN search tells you, I would suggest putting a monitor on it and looking at the coolant to oil temp delta while driving it with a load or up a long grade to see where the temps settle out. I think Auto Enginuity or Torque Pro software can do this, but don't quote me on that. I'd find somebody familiar with AE as it's likely that they can look at some other parameters that will help you determine the health of the engine.
It's a really good price, but just know that you'll have to at least do an oil cooler and the big flush as a starting point. That's the best scenario. The worst scenario is that you have to do everything and that will make that initial price seem like chump change.
It's very unlikely that it has that many miles on it and hasn't had at least one cooler done already, but without any history it's a pretty huge gamble. I don't know if you can tell or not, but if there's any signs it's had the heads off or the cab has been pulled already, I wouldn't touch it. That's my personal opinion and that statement should be treated as such.
Good luck whichever way you decide to go.
