I had a 2005 GMC 2500 CCSB with LLY Duramax. It was fine for a while, but then need a lot of work. I love my RAM and damn glad I did some shopping and found RAM. DEF isn't an issue most of the time. Ask the owner if he's had any problems. Recalls fixed the issues.I've been a GM guy when it comes to daily / tow vehicles and currently have a 2006 LBZ 2500. Truck has been good but has issues developing as its tipping 250k. So I've been kicking around getting something newer and the ram 2500 was on the list but a 2012 because I don't like DEF and I would like to remove the exhaust restrictions to lower EGTs.
I'm currently in a 2016 laramie on a "extended test drive" since it came into the dealer I work at and so far its amazing. It is ten years newer though so thats to be expected. This truck does have some milage at 122k so I'm wondering in stock form whats to be exspected to blow up or go wrong with these trucks. I do know most all the service thats been done to it in the past and the previous owner wasn't one to dog on it. He had a issue with it getting stuck in 4wd so the dealer replaced the t-case and shift motor. They've also performed the water pump recall and some other things.
My driving style is just daily work commuter / pulling some wrecked vehicle into and out of the shop / pulling my offroad toys on a goose to play with a max load around 10k.