Wow.. Im using a 1/3HP West Coast Aquatics chiller on my 480 with a water volume of around 550, dual 480 watt MH's and a pletera of high volume external pumps that heat the water.
What is really boils down to is how much of a drop in temperature you require based on thr ambient temperature of your tank. Without a chiller my tank will peak around 85* regardless of how cool I keep my house. I keep the tank around 78-79 but factored in a target temp of 75*. A 1/3HP chiller will provide about a 10* drop at my water volume thus my choice of chiller size. It will actually go down even further without an issue but based on specs, that was the size chiller to get.
My suggestion would be to contact the chiller mfg of your choice and ask them what size you need for your water volume. My guess would a 1/4HP would be the max that you would need and I wouldnt be surprised is even a 1/5 would do the job. Unless of course you are doing a cold water tank or lobster tank. If you go too big, the chiller will cool the tank down faster but it will cycle more frequently which in turn might cost you more money in electricity.
One last thing to consider. I have the fortune of having my chiller mfg only a handful of miles from my home. Chilllers are not rocket science and for the most part, most of the reputable brands are very similar if not the same when it comes to major component such as the compressors.. I have not had any major issues with mine in the 12 years of service I have received from it but the one minor issue I did have, I was grateful to have the mfg in close proximity.
If you can find a decent chiller mfg that is close to you, that would be a plus but on the other hand, they are simple devices and as such very reliable.
