If you have a digital ballast, use Metal Halide for veg. Check under the DIY or lighting part of the forum. There's a lot of guides you may find useful.
I've heard that you can use HPS bulbs for the entire grow. If this is true; why would anyone get both MH and HPS?!?
It is true, MH is just a little bit more into the spectrum of veg for the plants. HPS is proven to work throughout though. Some systems with digital ballast come with both bulbs.
Well, this one would work with it. It says any bulb under 1000w (due to size) And this ballast runs MH and HPS. You'll just have to buy your choice of bulbs. High Tech Garden Supply Edit: Actually, for two plants a 250w would suffice, but I'd go 400w. I linked you to their 600w.
MH is more than a little more into the veg spectrum, it is optimized for that spectrum at 6500k. HPS can work for the entire grow, but since its spectrum around 2700k is not optimal for vegging you could get some internode stretching and less secondary growth. You can run both a MH and HPS off of a digital ballast. If you get an magnetic ballast (old kind) it will be made for either HPS or MH (and there are some that are switchable). If it is not switchable, you could get a HPS magnetic ballast and use a MH conversion bulb off of it ("conversion" meaning that is is specifically designed to run off a HPS ballast). Don't try to run a "regular" MH off an HPS magnetic ballast. In all cases, match wattages -- in other words, run only 400w bulbs off 400w ballasts, etc.
Depending on the type of lights being used wouldnt a metal locker create a pretty good heat issue. I would be more inclined to go with a grow tent.