The full apex will come with a ph probe, it will measure the ph of your aquarium water or calcium reactor effluent or freshly mixed water, etc.
You can also get an ORP probe for the full apex which you could use to control an ozone device, sulphur denitrator or just monitor your aquariums ORP.
The gold apex also comes with a conductivity probe to measure salinity.
Good choice.
Follow the instructions to the letter when setting it up, once up and running you will need to write some basic code to get it to do what you want, there is a good forum and you can prob get a lot of help here.
NEVER UPDATE THE FIRMWARE WIRELESSLY!
Most coding is easy, once you grasp it. 2nd peice of advice is to plan an available afternoon to set it up. Someone who has done it before, it would take all of an hour, but a new owner should plan an uniterupted afternoon. Also, plan on having a Apex friend available would help.