JMO, but I'd buy the 5th wheel first and then get the truck to pull it, especially if most of the truck usage will be to pull the trailer. Seems to me it's better find the trailer with the floor plan, build, and components you want, then get the truck to fit.
We've pulled our 5th wheels with a 2-WD truck for the last 13 years and never felt the need for 4-WD. It adds needless complexity, higher purchase price, and more fuel usage. Lose, lose, lose. My stepson just spent over $1200 getting his transfer case working. A good argument against 4-WD IMO. $1200 will buy a lot of one-time tows if you do need one occasionally, we never have had the need, but YMMV. If you spend most of your time on paved or dry gravel roads, not really necessary.