Personally, I would not bother with a Ca reactor until the Ca/Alk demand warrants it (if that ever happens). You can use a DIY 2-part additive or limewater for balanced Ca/Alk supplimentation in a large tank very inexpensively.
In fact, these days many people are automating addition of their DIY 2-part additive with dosing pumps and sustaining very high Ca/Alk demand setups without using a Ca reactor at all. IMO Ca reactors are sorta optional these days - not trying to discourage you from using one - just saying there are other options.
I just got my first batch of DIY 2-part stuff yesterday. I'm going to mix it up tonight.
I actually built the DIY calcium reactor through the MACO class a few years ago. I keep telling myself that, one day, I'll get around to getting the pumps, tanks, regulator, pH controller...blah..blah...blah. Too much $$$ if you ask me (especially when you figure that 2-part solution is cheap to make when you make it yourself).
My suggestion, save your money and don't get the reactor. Get some good quality test kits and check out how to make your own 2-part solution.
Adrian, and others, what pumps are you using to dose the 2 (or 3 ) part solutions? I like it but dosing daily is tedious, which is why I was considering a Ca Reactor. Thanks in advance!