If you have a wooden canopy over your tank, you can buy all the components and retrofit a new lighting system under your canopy. I did that for my reeftank, and it was MUCH cheaper than buying a pretty metal halide fixture. Also, with halides, your lights need to be higher off of the water than flourescent fixtures, so keep that in mind (I had to modify my wooden canopoy and make it taller). I'm no electrician by any means...hell, I'm dangerous with power tools, that's why I don't have any, but I was still able to wire a metal halide and PC retrofit system to go under my canopy. And most importantly, I didn't electrocute myself or burn down the building I live in. Installing fans in your system will definitely help keep your water cooler. I have 2x175 watt MH and 2/65 PC over my reef tank, and my temperatures didn't raise any at all. However, a lot will also depend on your room temperature as well.
If you do decide to build your own T5 fixture, many ballasts for PC and T5 bulbs can run either...it's the total wattage the matters. Metal halides are great, but T5s offer exceptional light output, and they run cooler and use less electricity, so there are fewer issues with your tank overheating. Good luck!