Just for comparison, I bought my 06 5.9 with only 37K miles on it last year for $30K. At that time they were going like hot cakes so I started looking out of state. I narrowed my search to west of Colorado due to the rust and build up on the undercarriage out east. I found mine in Arizona on ebay, looked at all the pics and called the dealer. Obviously the weather there is nice so the truck looked brand new, even the undercarriage. I eventually put a deposit down and flew down a few days later with my pops to check/buy it.
It was pricey but I didn't overpay what KBB had it at. The truck should last a long time and if I decide to sell it for some ungodly reason, the resale value is excellent. I couldn't be happier with it and I sold my jeep privately to use the money towards my truck which cut the cost down to $15k.
As for mods, I think they are awesome however I will not ever do anything to my truck. Sure it's fun to blow black smoke (which I can do without mods) and gain extra horsepower and torque but everything from there on out seems to go downhill. Once you replace something, something else starts to go out. I won't even do bigger tires. I have zero mods and I'm at 23-25 mpgs (half city and half highway). I did however tint the windows but it seemed to have thrown off my blinker fluid light...