Planning that far out, you are way ahead of the game! Too many people get in a rush, and make mistakes.

Other than a new bulb when you need one, I wouldn't spend any more on the light.

As long as you leave the doors open, the heat shouldn't be an issue. Will you be able to leave it open during flower? If not, will you be around twice a day to open/ close it?

600 watt HPS is the most efficient of the HPS, so you're good there. Personally, I'd hang it as high as possible in the tent. That way, as the plant grows closer to the light, the intensity goes up.