Not sure where you are located or what kind of weather you plan to brood in as to the future. My biggest challenge to brooding outside is temperature swings, but I brood when it is below freezing or in the heat of summer. I've had temperatures swing from below freezing to in the 70's within 36 hours. To me, a brooder needs to provide a warm enough spot in the coldest of temperatures they will see and a cool enough spot in the warmest temperatures. I find mine are really good at selecting where they want to be if given the option.
My brooder is 3' x 6', I can easily keep 25 to 30 chicks in it until they are 5 weeks old. I heat one end and let the other end cool off as it will. Some mornings I've had ice on one end but the chicks were toasty warm on the other end. In the heat of summer they spend a lot of time in the cool end. I use heat lamps and vary the wattage of the bulbs as needed but heating pads and heat plates can work great too.
My brooder is elevated with a 1/2" hardware cloth bottom. The poop falls through into bins underneath. That keeps the brooder dry and clean. In colder weather I put a piece of plywood on the brooder floor on the heated end to help keep that portion warmed. I just tilt the board and maybe scrape it to clean if I need to, very easy. I've used it as a broody buster, I've had as many as three broodies in it at one time. I've isolated injured chickens in it. I also built my 4' x 8' grow-out coop at the far end of the main run elevated and with a wire floor so I have a back-up broody buster and isolation area if I need it. I like to build as much flexibility into things as I can, makes it a lot easier to manage things as they happen.
