The dead batteries would concern me, especially since it had aftermarket wiring. If I was looking at it I would have a garage check it out.
One of the old rules of thumb that my dad told me was that you don't buy a ford from a chevy dealer. People who were happy with their vehicle tend to trade them in for one of the same make. It doesn't always apply, but it's worth considering.
As far as only one head gasket being repaired - the 6.0 is the most recalled engine that Ford ever offered. They averaged $1000 per unit sold on recalls. They got downright stingy on doing the work on them toward the end of the run. Mine went back numerous times for overheating and they kept replacing thermostats and degas caps. The real problem was a blown head gasket that could have been easily diagnosed by looking for combustion gases in the degas bottle. They strung me along until the truck was out of warranty.
I finally bit the bullet and paid a shop $8000 to bulletproof mine. I loved the truck, but was having problems with the engine and couldn't afford a new truck.
The story I've been told about the 6.0 is that Navistar designed them as a 300 hp engine. Ford tuned them to 340 hp to win the HP war with chevy and dodge. Ford sued Navistar over the failures and Navistar responded by saying the Ford had modified the tuning and it wasn't Navistar's fault they blew head gaskets.
If you don't run an aftermarket tune, put a coolant filter on it before the coolers clog, and use good quality oil and filters they're pretty good engines. These are things I found out the hard way. I put a hypermax econ tuner on mine and it advanced the fuel timing. The truck managed to stay together for about 60,000 miles with the tuner on it, but eventually it stretched the TTY studs and blew a gasket. It blew out the coolant and I took it to a little hole-in-the-wall gas station where the technician put the wrong coolant in it. That coolant formed silicates that clogged the oil cooler and ended up causing the EGR cooler to fail.