Buying a diesel! Need opinions
 

Hey everyone, I'm new to DB and this is pretty cool.
I need some advice and someone to point me in the right direction. I'm looking at buying a diesel. Will be towing a camper 7,000 lbs across the country for work.

I'm looking at a
2007 3500 LBZ auto crew cab long box with 260k
99 F350 7.3 6 speed single cab reg cab 179k
95 2500 12 valve 5 speed Cummins 280k

All are nice trucks for the years with upgrades and no rust!
What should I buy especially for towing across the open road and through the mountains. Mpg, reliability, experience opinions please.