You say you have 2WPG with 30 watts... that means a 15g that's about 18" long, I'm assuming. 24" long bulbs are 20 watts each, so a dual tube 24" flourescent rig will get you 40 watts. The smallest Power Compact strip you're gonna find is 24" as well, and it puts out 55-65 watts.. depending on the brand. For the cost of materials, if you know what you're doing, you can build a custom hood with few cheap ballasts and get 40-60 watts easily. The point being, it's not gonna be very expensive.... no way you'd need to get into Metal Halide bulbs... they're 175 watts each.
As for the bulbs and spectrum, Osram is Sylvania to us in America.... and I haven't found a Sylvania bulb that's the correct spectrum, but then again I haven't looked very hard either. I'm sure they make something in the "daylight" spectrum. I picked up GE 24" 20w 6500K flourescent bulbs at Lowe's the other day for about $4 each.... so it is most definitely possible to use standard bulbs instead of the expensive Coralife/Hagen stuff.