OP, just a thought... if you buy a nice used rig and blow your budget, but take it across country and sell it within the year or the next year, you could probably sell it for about as much as you bought it.
DrewE had a great post. Something else to consider is towing a vehicle behind you, called a Toad. Unless you buy a small enough rig to drive around town AND are willing to unhook every time you need groceries or want to go to a tourist attraction, you will want a small car behind you that you can use for local trips.
Good luck, a cross country trip is an amazing good time!
