advise on buying A calcium Reactor

[Gools]

Looking into getting a calcium reactor. But I don't know anything about them. What should I look for when buying one? Are there things that aren't neccesary, that a dealer would try to talk me into buying? Do I need to have a digital ph meter? All help will be greatly appreciated. Also if you have any reading you can send me on these that would be great. Thanks. Gools