You/we need to know some details about the fifth wheel you plan on buying before you decide on a truck - especially if you're looking at 3/4-ton trucks. Most 3/4-ton trucks will hit the limit on payload/rear axle capacity long before max towing capacity. Search for and read the threads on this or other forums about weights and capacities for towing fifth wheels. Later model trucks have higher ratings but, in short, you'll need to stick with a fifth wheel around 10,000 lbs. GVWR or less to be comfortable with the pin weight on a 3/4-ton truck... especially if you buy an older truck. There's not much price difference between a 3/4-ton and similarly equipped 1-ton truck of the same age and mileage.
Rob