We've been living full-time in our 5th wheel for 2+ years. Spent several months researching before we purchased. There are 2 basic categories of 5th wheels: I-beam frame (95% of the market - Grand Design, Montana, Landmark, etc.) and custom box tube frame (far stronger, very expensive, and sort of like roll cage construction - New Horizon, Mobile Suites, Spacecraft, Carriage, Excel, Teton Homes, etc.). The I-beam frames are almost all built by one company - Lippert. Further, there are standard-width 96" trailers and 'wide-body' 102" trailers. Those few extra inches make a HUGE difference inside.
Even with a very large 43-footer, it will be tight with that many people. We've had 5 adults and 3 small dogs in ours for a few days when the kids are visiting and it is a PIA with people sleeping on couches and on the floor
Used is the only way to go. We bought a like-new wide body tube-frame trailer for far less than the cost of one of the cheapo plastic palace I-beam trailers brand new.