The superdutys of 05-07 are good trucks. Our company has had 4 of them over the years (F350's SRW), they have pretty good ride, lots of room in the cab etc. But I would definitly be cautious of the 6.0L, they have good towing power but have been prone to head gasket failures, and injector problems. We had one break a tip of an injector off and rattle around the cylinder

and I've talked to many others with similar problems (but some were also abused). Unless you a good history on the truck I wouldn't go 6.0L.