Was given an offer I couldn't refuse for an '06 almost six years ago. Did a lot of research on the 6.0 and found a thread on some truck forum from a retired Ford engine assembler. He said the blocks they got in box cars would be put on engine stands to bolt on whatever. When they rotated them sometimes casting sand would come out. International made them for school buses & oil coolers, etc. weren't added. (Researched how well these did in buses and found out they were seriously great!) He went on and said how since Ford never flushed the blocks out, anyone buying one should right away. My new/old truck only had 28k miles on it so I did that at a Jiffy Lube. Coolant coming out was actually grey in color! I added an aftermarket spin on coolant filter and have had none of the problems others claim to have had at all in 60k miles.
What the Ford guy went on to say was that this casting sand (silicate) would get into the little tubes of the oil cooler and such, and they'd clog up. This would decrease oil flow, increase engine temp and do nasty stuff. It's also why there were no major problems with that engine in school bus applications.
Most of what you hear about the 6.0 is second, third and sometimes 18th hand in my book. I've heard directly from only one person who actually had a blown head gasket, and it wasn't due to 'stretched bolts' but running with turbo over pressure and engine over temp. Several have told me they got over 200k miles before selling their trucks. One claimed 400k and then had to junk it after an accident.
If you're getting a good deal, it's been well taken care of and low mileage, I'd go for it. Check what the coolant looks like though and get a power flush if the previous owner hasn't done that. Not cheap, but worth it.
2010 Montana 3665RE, wet bolts, etc.
2006 Ford F350 Diesel Dually, air bags, Softopper, Aeroshild, etc.