For two plants 400 watt hps minimum. Reflective coatings on sides, side lighting cfl's, ph kit, how are you going to control the temp. that hps is going to put off some heat?
I'm growing in a basement in Northeast USA and I have found I need to keep a heater set to 74 degrees to keep the temps up. I have small room closed off with 4 CFLS running. Once I go to HPS the temps will rise. At night I hear the heater kick on A LOT so you might have problems with cold temps.