50 degrees fahrenheit is on the low end of the advisable working temperature, although some paints allow temps as low as 35. Read the label, but in general, when you are comfortable outside in normal clothing,the paint will work well.

A quart of paint will cover 70 t0 80 square feet of surface, so it should be plenty.

You can roll and brush or just brush on such a small project. If you roll, brushing out the roller texture will give a better surface appearance, especially if you are using a paint with a higher sheen.