Here just some general information regarding how many gallons of paint you will need to paint a room.  Keep in mind that these are just guidelines that will help you determine the amount.  One gallon of paint roughly cover 375 to 400 square feet.  Take your room perimeter multiply it by the wall height which gives you the total square footage.   To play it safe, divide your room's square footage by 375 to get the number of gallons.  Chances are you will need two coats, in that case, double that number!    From all of my painting experiences, buying a decent brush does make the difference.  I am a huge fan of the Purdy lines of brushes, they do not loose their bristles or shape...even after repeated uses.  They dry quick and I usually rewrap them in the original packaging they come in.  Helps keep their shape.  Consider spending a bit more up front on supplies and the project will go without a glitch!