chereau refigerated trailor

Hello my name is Phil Jones im thinking about buying a chereau semi trailor refigirated unit they all have good revues
however im a little confused on when the specification changed with my budget im looking at something between a 1997 and a 2004 model ive been told the insulation on some models is thicker than others and some trailors are stronger than others due to the base plate being thicker
when did this change ? can someone please send me a little more info

thanks regards

Phil Jones