I bought mine for less than $10,000 in 2013 but I'm sure my total investment in the vehicle now is over $15,000. I've done a lot of repairs and part replacements. Some out of need, some just to make sure I don't have an issue on the road. All of them are old now. If you buy something that old, just expect to spend the time and money to get it into good condition. Even if it was well cared for, the previous owner probably wasn't fixing things with the plan to keep it long term, so it might need a bit of work to make it truly reliable. I plan to haul my trailer out west this summer for 16 days and 6,000 miles and I'm not concerned that it is 14 years old and has over 150k miles on it, but I've replaced everything that shows the slightest sign of trouble or is a likely candidate for trouble.
I've towed with an '06 Duramax 2500HD also, much nicer. Honestly in the midwest, it doesn't matter much, but when the hills get bigger and more frequent, the quiet cruising of the diesel is really nice on a longer haul. Almost always stays in 6th gear.
Good luck with whatever you choose!
