The units themselves get hot, but the rest of the room won't heat up comparable to a sauna. But if the room is small enough it will be a nice temp, but not hot.
I have two 150W'ers over my 6 foot and it's just enough spread-wise. The light easily penetrates to the substrate at 2 feet tall (plus another ~40cm (distance from light to water's surface). One of the best things you get from MH lighting is the ripple-effect when the water moves on the surface, it's pretty cool to see.
Regards to using CO2, it would be better if you did, but it can be done without it. I had a tank in high school that had a single MH over it, without any ferts and only DIY CO2 every now and then, and I had great growth.