Sup Arthur, definitely size up. If you go true-to-size, the back and shoulders are gonna bind up like crazy when you move around, especially over an undershirt. Sizing up gives you that extra chest room and keeps the leg cuffs sitting right over heavy boots without pulling up when you walk.
As for the heat, heavy-duty Workrites will have you sweating through your soul in a couple of hours indoors. Lighter blends are way easier to survive full con days in, and they take acrylic washes or fraying super easily for weathering anyway. A solid option to look at is this Michael Myers costume, as it has a good base look without feeling like a total sauna.
Good luck with the build!