1. I suggest you to put the tank outside. The reason is, the pressure in the tank will get higher at harm temperature, so you will benefit of more co2 before the tank seems to be empty. And another thing that is good is that you dont have to open the door to adjust your regulator that might be hidden behind kegs.
4. No, because I'm not finished yet with mine, but planning is the key. For me it's a long process, I want perfection. I found a danby minifrige with a digital thermostat accessible from the top front, it show me the temperature in the fridge, it's awsome..
Another hint, if your a freak like me. I found that pvc parts from electric outdoor boxs can be usefull to insulate the holes in the fridge some of these part can be connected through the foam with rubber rings and let a 1/2'' hose fit perfectly.