Suggestions for older pickup to tow 18k lbs 40'+ FW

IMO you're pretty much out of luck trying to find a truck at that price point. An older truck would have to be a dually and a diesel to boot, and even then I think you're going to find that an 18,000 lb. 5th wheel is too much trailer for what you're considering.

I pull a 15,000 lb. 5th wheel with my 2001 Dodge dually and I'm right at the edge, and a little over, when it comes to capacity. You need to either look at newer, and more expensive, trucks or a smaller/lighter 5th wheel.