14 bolts come around every now and then if you look for them. Air bags are really the right way to do it, they install easy, and they can be adjusted for the varying loads. The biggest improvement you will see is when you are empty. Use a york 210 compressor and a air dryer and you will not have to think about it for 10 years. You will need a break controller obviously. Tires can be bought at cost if you talk around enough.
You can waste your money and go with the cheaper options or do it right and end up spending 4k. It all comes down to how much you like the truck and if it is worth it to you to do the conversion or switch to a higher rated truck.
A good trailer and good loading principles will make more of a difference in how it tows then the weight rating of the truck used. In your case, because you do not have the option of a fifth wheel a triple or double (spread out a few feet) axle may work best.