My opinion.... Do some (ok a lot of) reading and make an informed decision. There are "stickies" in the 6.0 General Discussion forum and in the 6.0 Tech Files forum that will help answer some of your questions and give you a ton of info. Oil coolers, EGR coolers and coolant are the big ones to read up on. Think about what you want the vehicle for, (towing, family hauler, weekend truck or daily driver, etc.) and decide if this is the truck that fits.
I love mine and hope to have it for a long long time. For me I needed something to haul 6 kids, gear for a 1000 mile round trip and pull a 19' boat. The Ex is the biggest I could find and the diesel... well I'm hoping it'll last a long time.
Answers to your questions...
Is the 6.0 a time bomb... YES!! But only if you neglect it.
Can you add power... YES!! But like anything if you go too far you'll pay for it. With a custom tuned SCT it'll be fine as long as you don't neglect it.
As for the gas mileage... I get a combined average of 16 MPG but that varies with my use of the skinny pedal. I can get 21 or 22 on the highway if I'm conservative or I can drop that to 15 on the highway by driving 70 and passing everything at WOT.
I couldn't think of a way to work in a comment about neglecting it for that last one. The 6.0 needs to be taken care of. If you neglect it, it''ll let you know and it'll be inconvenient and expensive.
Good luck with the search, enjoy the reading and if you have any questions just ask. There is a ton of great knowledge on this site.