I did a lot of research when I put my 320 Gallon system together and
Emperor Aquatics puts out probably the best quality ones, but that's the easy question.
It depends on what you want the unit to do? If you want to sort of keep the water free of water born algaes, by one rated for your tank size and it will be several hunderd dollars, but if you want to do do protozoa, bacteria, viruses, etc you will be looking at several thousand dollars.
The size of the tank is only one parameter. The others are what do you want to kill, your flow rate through the system, the multiplication factor of the lifeform you want eliminate, and the wattage. With other types of filters you can slow the flow through the system and the growth of say phosphates will accumulate so slowly that you will keep system pretty much near zero.
With life forms their mulitplication factor will way outstrip this method, you must have a large enough unit to zap everything you want to kill the first time though. see this thread:
http://www.reefcentral.com/forums/showthread.php?t=2013892
Also keep in mind that you cannot harm non-waterborn orchanisms such as ich with it.
