There's lots of info on RC and nano-reef, about people using shop-type mh balast and housings over aquariums. It CAN be done, but it requires a certain amount of research. You have to make sure you're getting the right wattage ballast, and you have to make sure that the fixture will accept the bulb.
incysors right, if its an electronic ballast it will fire almost any bulb of the same wattage but if its a magnetic ballast thats a different story because some bulbs have an internal ignitor and some require a seperate ingnitor, thats the difference between European bulbs and American bulbs.
do you already have the ballast? if so check the ansi number on the label
There's a single 250w 10k MH retrofit kit on this page for $150, already all wired up...All you'll need to do is make a reflector and mount it. Not quite as cheap as the DIY you posted....but with a better bulb, and less work.