Will I need a HPS later
 

I bought a 400w MH ballast/bulb/reflector for the 11 plant system. Seems pretty nice, the heavy ballast is separate from the fixture (I don't have to hang it.) However, I'm a little uncertain about their applications. Will I need a HPS later in order for the plants to bloom? I thought I'd be ok with just the MH for vegetables, herbs etc. Will my flowers be able to bloom under MH (carnations). Lighting is big $$ and I didn't want to invest in both. I live in the northeast and am planning on starting indoors in the winter and than moving to the plants outside on the deck in Late Spring and Summer.

New 2 Hydro Holly

Yes MH is mostly for Veg. growth stage but since you are putting your plants on the deck in the spring that should supply your plants with any missing orange light spectrum you might be missing. Plants will grow and bud under either MH and HPS, for those on a budget. It's just logical that they would grow better (faster) under the full spectrum of light needed for healthy plant growth.
Ron