The plants look as healthy as any as I've seen. Great job so far. The flowering phase is what separates the men from the boys though. Keep a sharp eye on pH, don't overfertilize, be patient, and remember........less is more. Good luck.
Yep. If you've got the money for an HPS conversion bulb though, you should definitely spring for it. Plants love that spectrum during flowering and will really fatten up if you do.
Yes. Just look for what's called a 1000 watt (or 940 watt) HPS conversion bulb at your growshop. Tell them you have a halide ballast and they should be able to hook you up. They're usually around $100-140 though.