Just plugging a 2005 XLT, 4x4, Super Cab, long box into Kelly Blue Book shows a retail value of $30,175 using my own zip code. The upfitter switches and the TowCommand system were worth about $350 as options. So, that sounds like it is in the right ballpark, but you may want to hit Kelly Blue Book, and check what it is valued at for your region.

If you do get serious about buying, I would strongly suggest getting the dealer to put in a stock air intake system. Work it in as part of the sales deal, and share as little of the cost for the intake setup as possible. If the dealership wants to make the sale, they may very well throw it in, and absorb the cost with the sale of the truck. Incidentally, there are tons of posts throughout the 6.0L forum here on Ford Truck Enthusiasts regarding the 6.0L motor & aftermarket air intake systems.

Good luck!!