You have to buy a 400 watt MH light? Then why not do even better and buy a 400Watt switchable ballast then you can run a 400w MH for veg and HPS bulb for flowering.
Check ebay I bought a 400w MH/HPS switchable ballast(Sun System) 1 MH bulb, 1 HPS bulb, Sun Reflector(Bat Wing),1 Roll of mylar 100ft and the Mogul socket all for 309.00USD shipped.
DFG


P.S Sometimes you can find better deals buying pieces instead of the systems.
DFG