I will say a lab grade pH probe is pretty much all you should go with regardless of what you are monitoring or doing with it. The price difference is minimal and the difference is huge. It's not just the quality, but how it is physically constructed. BRS did a pretty good video explaining the difference quite some time ago:
https://www.youtube.com/watch?v=P-BFntfiKTs
That being said, if memory servers me correctly it was actually cheaper for me to get an Apex with normal probe and buy a lab grade separately. Neptune's probes aren't special and the Apex uses a standard plug for its probes.
As for the gold version, that is really up to you and what you want to monitor. The only real thing you get extra is the ability to monitor salinity. At my house, one tank could use that and one could not. My opinion is that if you do not have an auto top off on your system, salinity is a waste of money to monitor. You will only change through evaporation and manual top offs.
I also agree with the Apex being a great addition. I have one on each of my tanks and wouldn't want it any other way. It does, however, add a bit of complication to the tank.
As far as what I do with my Apex, the short answer is everything. I will not buy anything for my tank that is not Apex compatible. I monitor temp, pH in the tank and calcium reactor, monitor temp, control lights, powerheads, skimmer, skimmer cleaner and heater. My larger tank uses the Apex for top off and once I get a bracket built my autofeeder will go on as well. I also have mine sending out texts and emails if certain parameters happen. If you know what you are doing or someone who does there is almost nothing you can't have the apex do for you.