im gonna try and answer your question in the order you asked. let me start by saying that i am very familiar with all the equiptment you asked about because i just finished building almost the exact cages that you are describing. i built 5) ball python cages at 2x2x4 and 2 boa cages at 2x2x7. ok RHP 80watt for 4x2x18 is the perfect size( a bit over kill but no problem on a thermostat.120 for 6x2x18 is also a good size. i use 88w in 4x2x2 , and 135w in 7x2x2. my temps on the hot side are 86deg. ambiant ,92 deg baskingand 79deg. cool side. Mount the heat panel off to one side, about 6" from the edge this will create a good gradiant. Either proportional or on/off thermostats will work but proportional will give you a much more even and accurate temp. Doors.... definately glass sliding doors. Swing doors tend to want to warp leaving an easy escape, also its nice not to have to keep moving out of the way of swinging doors! Thermostats....the temps would be set the same in either proportional or on/off mode but when you use on/off mode its the "swing" setting that is important.On herpstats you can set how much of a temp swing you want. I personally have my probe set on the cool side at 79deg. this way i am able to give my animals optimal heat gradiants.They will self regulate if you give them the right temps. Treating the wood.... use several coats of polyurathane! it is by far the best thing to seal out moisture.Caulk all seals after poly dries. allow enough time for caulk to cure and all fumes to ventalate