You always need ventilation. The primary consideration here is that HID bulbs result in heat, which usually has to be dealt with (especially in summer).
Like I said in your other thread, and as the others alluded to, you would need multiple ballasts to run all three bulbs. Once again, this would also be less efficient. Three 150 watt HID lamps are actually drawing roughly 500 watts (~170 watts per ballast x 3).
The lower wattage lamps are less efficient in terms of light output as well, they produce less lumen per watt. I really don't think you're going to be able to find a dual 150 for this reason, it wouldn't be practical. From HTG Supply, it looks like the 150 watt HPS system with cheap reflector and bulb included is $70, granted that you already have bulbs.
Again, the lamps put off a significant amount of heat. A single 150 or 250 watt HID lamp will get quite hot; but this is more simply dealt with and heat can be controlled with a couple fans and proper ventilation (exhaustion of hot air somehow). Once you start talking about multiple bulbs (or higher wattage bulbs) you need to consider the additional heat that those bulbs are going to throw off (not the mention the heat produced by the ballasts), and associated equipment for cooling everything, as well as the ease of the whole setup.
You could get a single 250 or 400 watt HID system for under $200 from HTG Supply.
http://www.htgsupply.com/Category-Grow-Lights.asp
You can save money by just getting a standard cheap winged-aluminum reflector, but it really does help to have a higher quality one. Some are totally enclosed, and have a tempered glass lens, typically also featuring flanges for air-cooling. The reflector will also affect lighting coverage.
Use the CFLs as side lighting.
