It depends on what bells and whistles are your must-haves. I went the buy for the future route and also went with a 3/4 ton diesel. However, I went backwards in time and got a one-owner rust-free '09 Duramax (no DEF) LTZ . The truck spent its entire life in TX and had full service records. It also already had a 4" Rancho lift and aftermarket wheels and tires. The dealer installed new factory headlights and tailights which was a nice touch to replace the old cloudy units. Inside, the truck looks brand new other than the typical crack in the dashboard these are known for developing. It was top of the line for its day, but now is pretty domestic with tiny backup camera in the rear view mirror, digital Bose stereo without touch, Apple CarPlay, etc...
I spent the time and money to bullet proof the emissions system and also installed a 5 position tune and lift pump. I also deleted the factory fuel filter and installed ported fuel rail fittings and a race plug to prevent common fuel rail codes.
I upgraded the 20s and 33s to 17s and 35s. Finally, several weeks back I had my local body guy bulletproof the bottom of the rockers and the quarters in front and back of the rear wheels with bedliner spray. Since he already had it taken apart, I had him install a set of subtle fender flares (no fake bolts) and mud guards. I had it undercoated and inner fender coated right after I bought it last fall, and since this is my "forever" truck my upgrade objectives from the beginning focused on capability and longevity.
I pull a 17' boat and occassionally a 20' tandem trailer and the 120 tune is my "daily" driver. With this combination, as long as I keep the truck under 75mph, I average around 18-19 mpg and around 15 mpg towing.
Right now I'm around $32k all-in with the upgrades and the truck has 199k miles. As long as I am conscientious with the cleaining and maintenance, I should expect 500k or more without issues. So far, I absolutely love the truck and its been very reliable with a trip to NW WY and a couple trips to CO under its belt.