If they're only asking $6900 for it, I'd let it sit, something doesn't smell right. The Ford 6.0 can be a very good engine, but they take a lot of flak because you only hear about the bad ones.
They are maintenance intensive, which a lot of people don't keep up with. Fluid changes must be done on schedule or the results will get into your checking account real quick. Some of these engines have gone over 500K miles, some blow at 80K. Oil has to be changed at 7500 miles or less, fuel filters have to be changed every other oil change. If it throws a check engine light, it shouldn't be ignored to see if it will go away. They really don't like short trip driving, it's hard on the turbo, the batteries and they don't like to idled for long periods.
I inherited mine from a relative, it's almost completely stock other than the EGR delete and a coolant filter. I've done a few preemptive modifications to it to hopefully prevent a major blow up, maybe to the tune of $300 or $400 worth of outlay. They also need to monitored so you can shut it down if you see something not right.
Mine currently has a little over 118K on it and the only thing I've really had to do to it other than fluids and filters is one glow plug, which cost a whopping $10.
In other words it's not an engine for a casual user, short trips will kill it slowly, it needs to be taken care of - so for a large percentage of the drivers out there, it's not the one they need or want.
The 7.3 is a pretty good engine and doesn't require near the amount of preemptive care the 6.0 does.
Ford also put some V10 gasoline engines in those trucks, seems most of them had manual transmissions, but they're hard to find and you wouldn't believe how fast they can drain a gas tank.
I'd suggest a F350 with the 6.2 gas engine in it.