Buying used also saves a lot of money. A camper just a few months old will lose 10-20 percent of its sale value.
Buying new or used has its pitfalls. In the used category leaks are a big issue and its impact changes with the type of construction.
The "tin and wood" construction is cheap and heavy. It is also the most sensitive to water leaks. It is also a lot cheaper to fix than composite bonded structures that are both light and expensive.
Water rots wood quickly. In tin and wood trailers it is not always easy to detect. Water might travel a considerable distance from the source to where it may pool. Fixing any leak and the damage can be extensive, but at least for tin and would the skills required are not extreme or that special. Chances are if you repair them yourself, both the materials and repair will be of better quality than the original build.
Detecting such damage can be done with your Mark I nose (you can sometimes smell the rot) and pressure from your hand or foot. Pay attention to surfaces below doors, windows, and other penetrations. You foot may detect a sag. Be sure to walk the roof and check for sags or signs the chalking was not kept up with. Check the decals. Decals can indicate damage.
If you can get dealerships to compete for your business you can save a lot. For any used TT you consider get the wholesale value of the trailer with whatever optional equipment was installed. As you approach that number with the salesman, you are getting about as low as they will go.
When someone buys a new trailer and has an old one for tradein the wholesale value is just about as much as the dealership will sell the unit for. Actually they can sell it it is less than that because they are offsetting possibly buying the used unit for too much in the price of the new unit. Still it is important to know that number so you know who is giving you a deal and who is not.
I agree with the other posters here. The manufacturers will say their unit sleeps X. My old Trailmanor was rated at 5, but with 2 the other three better be gnomes.
I had my daughter and my grandson stay with us once in FL so we had 4 in the Trailmanor (TM). The only reason we got along without killing each other at night is that I was married to the one in the bed with me!
What I am saying is if you think you are going to inside the unit more than outside take a good look at the living space. You have a bed to setup every night, and when you set it up you are going to lose table space.
There is no reason 4 people cannot stay in that unit, but you need to be aware just how tight it may be at night.