The 7.3L was a workhorse. Like everyone else said, the engine will be the least of your worries.
Diesels get a reputation for lasting 200k+ miles. There is some truth to that. Remember though the rest of the truck has also covered 200k+ miles. Rust, worn out axles, suspension, interior... The list goes on and on.
If the truck was driven in the South without the road salt we have here, and was well maintained, then 150K miles isn't too bad. We have 2 different Ford one ton Diesels at work with over 170k miles. They run very strong, but the rest of the vehicle starts to nickel and dime you after a while. They are not 7.3L's though, they are 6.0L's
Id look for a truck with less than 120k miles if you're looking at higher mileage trucks. I personally would probably never buy anything over 40k miles, but I drive too much a year.
2016 Ram 1500 Big Horn, "Katy"
2014 Outback Terrain 260trs "Alice"
2011 French Bulldog Shelter adopted edition, "Roscoe"
1982 DW, "Rachel"
2016 DD "Harper", the newest lil camper.