Can't answer the potting questions, mine are all in the ground.

I wouldn't use temperature as a guide for watering. If they are in full direct sunlight, they will probably need to be watered daily. If they are in part shade(more ideal), you might get away with watering every few days. How often you water though, will really be dependent on how well your soil/pot holds moisture.

You might find more info by doing a search for Hedychium coronarium

We grew them in pots when we lived in NJ (yes they have to be pot plants there)- I would use around a 5 gallon pot (they spread rapidly). They will probably need to be repotted/divided in the 2nd year.
Use any potting soil you have- they grow in anything- we have pure clay here in PR and they do fine.
Water when the soil looks almost dry- they can live in bogs, so they do like water. If they get too dry, the leaves will wilt a bit to let you know.
They bloom late in the summer- early fall.
Don't worry so much- they are really strong plants- very hard to kill (as long as they don't freeze).- Gwenn