The HID does put out quite a bit of heat, but is it going to be enough heat to keep your growroom at the desired temps and what are your growroom temps going to be like when the lights are off? This will depend on the area where your growroom is located and what sort of extraction system you've got. I'd do a test run over a 24 hr period and see what your temps are like before you decide to buy a heater or humidifier. I don't think the optimum temps change for the different stages of the plant. As far as humidity goes, i thought higher humidity was good for seedlings, but you should lower humidity in flowering as it may give problems with bud rot.

Good luck!