Can be a very reliable truck if treated right and taken care of. 6.0s like to be driven and like to work but not beat on without some thoughtful upgrades. Most everything you need to know was already stated. If you want to throw bigger injectors in there and a bigger turbo and aggressive tunes then get ready to spend money. Twice what that last build cost... At least... 6.0 can make great power and be efficient but it comes at a cost to reliability. Mine was making about 600hp with mostly bolt-ons and tuning by the previous owner and after 50k miles and some time at the strip, some heavy pulling it's got a new tranny and the motor is done at 200k miles. New motor for me. I'm spending a lot to build for power AND reliability.
05-07 are great years for the 6.0 and it looks like most of the weak points of those years were fixed (STC fitting, dummy plugs) so it looks like you've got yourself a pretty good truck there.
As others have mentioned verify the coolant type, highly recommended going to the red ELC coolant.
Add monitoring so you can guage the health of the oil cooler and other critical engine parameters.
No idea what the Banks "Big Hoss" kit includes but for $3500 I'd hope it was a lot! Probably a tuner, intercooler, air filter, etc. Take some photos of the engine bay and we can probably help you identify the various components that are aftermarket.
Beware there are some aftermarket components that are inferior to the stock stuff (air filters, oil filters, etc) so definitely post some photos of the engine bay.
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