I would purchase a 400w HPS ballast, that way you can use a MH conversion bulb for veg, then stick in the HPS bulb for flower. You get the best of both worlds, and only have to buy one ballast.

I would avoid 1000w, as 400w is sufficient for a grow of your size and uses much less electricity. Plus, you might have heat issues with the 1000w.

Height won't be a problem, as long as you don't let them grow 3 feet tall before flowering.