Dodge Diesel, which year or motor??

I'm trying to upgrade rigs soon and I'm looking hard at a Dodge 2500 Extended cab longbed diesel. Not towing anything extremely heavy just a 21' open sled with smaller camper hopefully. Occasionally a light car on a car hauler, say 7000lbs max. I just really like diesel for towing and better fuel economy.

I'm just trying to decide which year and which motor to go with. I know the 12valve motor is 98.5' and down and the 24valve is 98.5 and up. Rear discs started in '02. Would really like a 5 or 6 speed manual.

Wondering if anyone has any advice regarding the fuel economy between the 2 motors and towing or any advice at all. Not looking for max power, just a reliable strong setup that gets better than the 12-13 mpg I'm getting now while towing.

thanks
sean