although i've never run a lumatex, lots of people have good things to say about them. i know someone in the hydro industry who does a lot of equipment testing, i don't think you can go wrong with any of the ballasts on hydrofarm's site.
when i first heard about the dimmables, I didn't understand that you had to have separate bulbs for each setting... so if you want to run 400W then later 600W then 1000W, you need a 400, 600 and 1000W bulb separately.
although I think your way will be fine, you could actually get a cheaper brand 400W MH for veg and 1000HPS for bloom or 2 1000s or whatever for your bloom for the same price.
if you want to go the switchable/dimmable route, it's still a good choice according to many people here, but imvho, i'd go for more lights. it really depends on what your overall goals are here. How are the plants looking?