The reactor is just the reactor. You need a few other bits to get it going too. A co2 tank, co2 regulator, ph controller, and a parasitic pump. The pump seems optional, as a lot of guys still dose without one. Same as the ph controller, but that has gone more towards the mainstream of keeping your reactor stable.
Any of the mentioned reactors are great; Korallin, Vertex, Geo. There's a few more great ones, but these are the most popular in my opinion.
Also, size. You cannot over size a calcium reactor because you can control how the media devolves. So if you go big, say rated for 600g, you would adjust to a higher ph with the co2. The benefits of this is just not having to keep replenishing the media, since there will be more of it in a larger reactor.