How big is the brooder going to be for 300 chicks? I am guessing at least 4x8? If you are keeping it in a garage, shed or basement, you could build a 4x8 box with 24" sides using 2 1/2 sheet of plywood. I would make a hardware lid that fits inside the box, and place 8 heat lamps on top of the hardware wire. You can vary the height of the lid as the chicks grow, thus raising the heat lamps as they grow older.
If you want a free standing coop that will be outside, I think you are looking at a regular shed. Consider a shed frame kit that makes building the shed much easier. You provide the 2x4's, but all cuts are 90 degrees, no angles. Makes for fast construction.
You could also use a Rubbermaid watering trough or a galvanized steel one. The local farm supply stores use these.
Let us know what you decide to go with.