34mm thick cabins are available at Garden Street from brands you can trust. 34mm gives a nice balance between a thicker wall, and more competitive prices. 34mm walls would be cheaper than a 70mm thick wall, but provide a thicker finish than say a 12mm wall.
The thickness of the wall does not affect the insulation, and would still need to be treated routinely as with any wooden product. Try Protek for a great option for wood preserve.