Try and find one privately owned if you can(as there are plenty of them out there and you'll get out of paying sales taxes) just for a sense of how the truck was maintained by the previous owner.. 6.0s are great motors if well maintained. Ive owned 3 of them and love them.
If you are really considering buying a 6.0 do yourself a HUGE favor and purchase an edge CTS2 or even a SCT Livewire(little more spendy but will allow the addition of custom tuning after you buy the truck) before even test driving another one.. You can pick a CTS2 up for about 375 bucks and it is one of the best portable monitoring tools you can bring to look at a truck and makes for a great set of gauges when you do purchase a truck.
This will allow you to quickly plug into the OBDII port mount the CTS to the windshield and select the PIDS that you want to monitor. The main things are the engine coolant, and oil temps, your FICM voltages. But i just throw in the VGT cycle, icp, battery, and a few others just to check em out while I'm test driving.
Luckily I have one in my 7.3 that i use as gauges and I just remove it and take with me every time I go to look at a truck whether it be a 6.0, or 6.4.
If you google pressure testing 6.0 you'll find a quick method that will allow you to hook up a hose and gauge to the degas bottle and run into the cab to watch the pressures that build while driving and under WOT.
