Para45: A few words of advice, in no particular order.
Watch the GVWR of the trailer or 5th wheel, and don't believe anything the salesman tells you. You don't want to exceed the gross combined weight rating (for truck and loaded trailer). Or to put it another way, you may have have sufficient truck to pull it but do you have sufficient truck to stop it? Give serious consideration to an exhaust brake or compression brake, if your truck does not already have one.
As others have mentioned, where do you plan to store it? This is not just a matter of storage fees, but also security. Most of the storage lots that I have seen, experience periodic break-ins. That potentially adds the cost of pilfered items as well as the cost to repair any damage to the RV / trailer. Of course, if you are able to park it at home, then it is less of a consideration.
New or used, if you walk in and see or smell mildew, walk away. It will be very expensive to fix.
Consider how well the floor plan has been thought out. One of the niceties of RV travel is the ability to stop somewhere safe along the way to take a break and fix lunch. I saw a couple of floor plans where access to the kitchen and dining areas were blocked unless the slide was extended. It was a poorly-designed layout in my opinion.
Give some consideration to whether you might want to add airbags to your truck; or perhaps a airbag on the hitch. There are many different brands from which to choose.
Unlike automobiles, I know of no recognized "Consumer Reports" style organization that conducts evaluations on the various makes and models. Some years ago, there was a CD-ROM produced which offered a somewhat objective rating, but with the many consolidations and changes in the RV industry, I'm not certain whether there is anything which covers the current models.
If you were planning to live in it full-time, there are significant legal considerations but it sounds like that is not the case. Sounds like it's for weekend getaways and relatively short vacations.
OK, that was more than a few words; have fun in your search.