Good luck to you on whatever you decide to do..... I did a little looking and asked the wizard of oz and came up with no real major things on that engine....
Wickipedia
http://en.wikipedia.org/wiki/Detroit_Diesel_V8_engine has a few items that show up with age, crank problems, and head cracks, of course the one you are looking at is most likely on the well taken care of side.... People who "work" their stuff tend to take a little less than "good" care of stuff, especially when you get to watch them in person.... MPG seems to range from the lower to mid 20's depending on what it's in and how they use it.... Hey, you never really know what's going to happen anyway, a brand new car can leave you walking, you never know....
The stories go that that engine has the power of about a 305, better mileage, 20+ as opposed to about 15 with gas.
For 5K you can get 5 Dakota engines, or 3 and a take along mechanic.....
good luck ! ! & fun drivin'