will try to answer some of your questions but not nearly all of them you could use as much as 600 watts of lighting in that space for flowering you could use a 400 but it is a little weak for the space at under 50 watts per sq ft.
you want to use a hps for flowering for veg you can use flourecents for a few weeks or a small mh like a 250 or you can even use a hps and still have decent results.
mylar is worth using and will do a better job at reflecting light back to the plants.
all hid lights produce quite a lot of heat the higher the wattage the more heat they will produce and an air cooled reflector or tube hooked to a blower or fan helps to keep the temperature down also makes the light more effective as you will be able to keep it closer to the plants so before you buy a light you might think about how you will ventilate the light and room.
mh bulbs will not work on hps lights and vice versa but they do make switchable ballast systems that do allow you to change the bulb of the same wattage from mh to hps.