i think you would be good with a retrofit kit. wich will give you the bulb, socket(mogul), reflector, and ballast, along with the wiring you would need for it all.. then get or build a canopy and install them in there... thats normally the cheapest way to go. buying premanufactured hoods with halides already in them is mighty expensive..
depending on the bulb you got, be it a 10k or 12,14,20k bulb. would all depend on if you wanted actinics with it or not,, if you went with a 20k more than likley you wouldnt need any actinics unless you wanted it really blue.
im not sure of the demensions of your tank, but i would think you could get away with one 175watt halides and whatever you chose for actinics if you choose to have them.
i couldnt tell you a exact cost,,, i know my 175watt 10 xm bubls were 59.99 per bulb. i bought my reflector, ballast, and all with a old bulb off waterfaller1 a long time ago for 100 bucks. and my canopy ran me about another 60 in wood and parts. add the 2 4 inch fans for another 40ish.. and all together my setup cost me around $320 bucks, you would get away cheaper if you only needed one bulb.