You can only run one bulb off of one ballast unless you buy one of the special ballasts that will take multiple bulbs.
As far as the difference in bulbs, Its a crapshoot. I've bought both expensive and cheap hps 1000w bulbs and the only difference I've found is most of the time the cheaper bulbs last longer.
Hard to tell if you're going to have heat problems till you do it. Just build the room with ventilation in mind so that if it needs the extra ventilation you'll be able to do it.