Here is my 2 cents. As was previously stated, determine how hot the room you have the tank in gets so then you can contact a vendor and they can recommend the correct size.
There are 2 types, in-line and drop-in. The drop-in is recommended (not by me), so you don't need to plumb in-line. However, there are a couple of problems with drop in (I know, I returned mine), is that the hose is generally not flexible and this coil must be dropped into your sump. However, the bigger negative, is the drop in chillers are usually not as efficient to run as an in-line.
So once you decide on the the hp you need and hopefully, you decide on in-line, then you must remember that chillers are expensive to buy and then lot's of money on your electric bill.
FWIW, people are generally disatisfied with using DIY plans on the internet to build your one. You can save quite a bit of money, buying a used chiller from a Reef forum.
I am not a A/C technician, but when you look at the guts of these things, most of them are very simple. Chillers come with a controller or some do not, which means that you would need to buy a controller. Chillers are basically on-and-off units. So it's the controller that you set. There are dual stage controllers for controlling heaters and chiller and there are single stage controllers.
A common controller is Ranco or Aqualogic. They go from about $60 to almost $200 for a dual stage controller.
I bought my chiller 3 years ago on the cheap and since I made sure of the correct size I am totally satisfied. It's set and forget. Just make sure to oversize, rather than undersize.
In addition, oversizing just means that it cools quicker and can save money buy not staying on as long.
Some people also have used the controllers to turn on fans to blow accross the sump and then when it gets to hot, the chiller goes on. This tries to save on energy.
Good luck.