Hehehe, it's not at all cheap, but I have a Scientific Chiller 1/3rd horse sitting down my basement collecting dust . PolyScience Brand, we use 'em at work, have one that's been running for 3 years w/o so much as even touching it!
I'm sorry I can't offer any suggestions on what size to get, as I said, mine's collecting dust, I don't use it. I think most manufacturer's list what size they're rated for. I have however, heard that flow-thru style are supposed to be better than drop-in units. That's just what I've heard tho.
I have a 1/4 hpAqua logic cyclone on a 250 gal tank. It is adequate for the size tank but I do wish i had purchased the flow thru style instead. I find the length of the hose to the coil to be too short. Its not real noisy but you are aware of when it is running and they do put out a lot of heat into the room.