Look at what you think you want, rent a similar unit for week or so. You will learn a lot very quickly. In addition you can take the loaded for the road rig to the CAT scales and see what the true weight on each axel is, rather than a guess. You ideal TT may not be so ideal on the road.
When you buy make it absolutely clear that you are talking about the "cash out the door price". There are a lot of fees, taxes, prep charges etc. that can sneak in.
The industry builds about 8,000 TT and 5vers every week, they are not too likely to run out of supply.
Good luck.