I would look for a truck with less than 100K on the odometer and the large gas V-8 and the 4.10 rear end gears and the factory tow package (VIN lookup will tell if these are in the truck).
I would want to buy a truck from the original owner and someone who has all the maintenance records for the truck including oil changes and other routine servicing.
Watch out for any of these truck as most are likely to have badly worn tie rods and ball joints and for safety you will want to pay to have these replaced before towing a trailer.
The 3/4 ton trucks are heavier duty but also just plain heavier by 2,000 lbs. and that extra weight is not really doing much for you with a 8,000 lb. trailer.
Diesel is great for a 13,000 lbs. or heavier trailer with the added torque and exhaust braking but it will add $5,000 to the initial purchase price, require more maintenance, fuel will only be available at one in four gas stations when traveling, and many repairs can cost in the $5,000 to $8,000 range.
Gas engines overall (engine, fuel system, electricals, drivetrain, etc.) are more reliable than the diesel engine powered pickups. Folks often bring up tractors and their diesels but these are very different engines and they are maintained to a much greater degree than those used in pickup trucks and you might as well compare a Piper Cub to a Boeing 747.