Yeah, my 2001 Suburban is still going at 285k, original 5.3 engine and transmission, despite my attempts at abuse that have blown 3 rear ends due to towing really heavy things really far.
For the best absolute mechanical reliability and drive-it-into-the-ground durability, the 2500 trucks are it. Completely over-engineered components for the application. And no active fuel management system.
My 2008 2500 has 207k miles, is all-original, and has logged 25,000 miles of towing right at maximum ratings. I've spent about $300 in repairs over the last 7 years. Total. I fully expect at least 300k miles with minimal repairs/replacements.
The 5.3 in the 2000-2006 models was pretty damn good, but the 6.0 in the 2500 is even better. A legitimate 400k motor.
If you're looking for something mechanically reliable that you'd replace around 100k, most years would get you there.
I'm not a fan of GM's active fuel management, the cylinder deactivation for improved mileage. I'll never own one myself.
In that regard, I would avoid the 2007-2014 half-tons. And 2015 was the first year of the new platform, so I usually avoid those too. If cost isn't an obstacle, go 2016 or newer. If you're on a tighter budget, 2003-2006 would be my second choice. But you'd probably want to look at southern/California cars, since midwest/northeast models would probably have rust to some degree.
As much as "I" would love a 2500, it would be a hard sell for my wife's daily driver with 4 kids. Once my '01 Tahoe kicks the bucket, which I hope is a long time from now (210K miles) I will probably inherit the bourbon, so I'm thinking long term.
I'm in SC, so I'm only looking in states below the Mason Dixon line to avoid, as much as possible, the rust that comes from salted roads. I may even pull VA off my list.
Thanks for the super detailed reply!