I run a 700l SPS filled tank with just a kalk stirrer. I have 23 medium pieces in the tank. I manually dose for alkalinity daily which is what the CARX would do. I have incredible growth with my corals.
This pic was taken on the 25 August 08
This one on the third of Jan09. The raised middle bit in the top pic can be seen in the bottom pic
What you have been told is right....and wrong..
In the case of tony, he needs one.....
In your case, before getting to a stage where you would need a calcium reactor, you should focus your resources on establishing an ecosystem ready for sps.
Depending on your current bioload, I would doubt if your current skimmer is adequate for a system of that size when it reaches the point of needing a calcium reactor.
Adding Coral is a broad term...What type of corals do you intend adding?
I've been doing fine without a CARX for a year now. Manually dosing is like the balling method without the pumps. I know my daily consumption and I supplement and test daily to make sure thing are OK.
If you couldn't be bothered to test daily and are "lazy" then a CARX with a p.H controller is the ticket or the balling method which involves an initial amount of work, tweaking and testing to get the daily consumption