I checked and since you live in California, I would strongly recommend you get a refrigerated container. These are well insulated and are most probably the only thing saving you from turning your lab into an oven during the summer months.
For condensation, insulation and good ventilation should take care of the problem. Here in Canada, most of the moisture comes from the ground so unless we have a full blown foundation with drainage, the only way to build to avoid such problems is to build up from the ground.
For a layout, the most useful and nice is usually to have the dry side opposite the wet side. This saves you a ton of steps. I have a linear layout in my darkroom and it works only because it's less than 12 feet long.
For ventilation, put a hood over the sink. It's definitely the best way to save your nose and lungs.
Depending on the size of the container you're going to get, there are so many possibilities!