I have a 6 year old alutec with no body panel problems, so to me the construction is proven.
Unfortunately lack of attention to sealing the floor joints has lead to leaks onto the WOOD floor, ok it was sorted in the first 18 months, but only on the third attempt. Not all caravans were affected.
Bailey quickly changed to a U2, but again leaks to the WOOD floor from primarily shower leaks caused some owners problems.
On attempt three Bailey brought out a U3, while some would say this is the best yet, my opinion is the U1 was easily the best, more equipment for a start.
But again because Bailey decided to introduce a ROOF joint, not present on a U1, this in turn has lead to leaks into the caravan, it also appears that all caravans can be affected as the joints are being replaced at service.
Moving onto Swift HT construction my next caravan on order. The construction is simular to the U3, but the floor is WOODLESS, being a composite construction.
It has however only been around since about 2015, so no fully proved long term.
The front and rear panels are also GRP with complicated to me joints to the side walls and roof, so only time will tell.
Positive points are very few if any reports of leaks through the body, and on the top models even the interior furniture is a part sandwich construction, with a woodless interior.
Price wise i think you are looking at an Alutech. But ensure you use a damp meter to all the floor area.
My caravan is on sale on this site if that's the layout you want.
2011 - 2014 is precisely the period when most of the UK caravan producers seemed to go through a very bad patch for water ingress. I'm not saying they were all leakers, but just be careful what you buy. I would want to see the service report for each and every annual service before a thorough damp test by my own NCC mobile service person. I don't know whether manufacturers warranty is transferrable to a second user
My Bailey came with a 10 year body warranty, later models only 6 with an option of 4 more, fully transferable providing service has been maintained.
Don't even consider SOLID.