It really just depends on what all you want from it and how much you’re willing to spend! I do maintenances in my area and have multiple accounts with all the bells and whistles: auto dosing, testing, top off, water changes, feeding, flow monitoring for UV, as well as all the probes just to name a few things. I only mention this because what you can do all depends on what model you get. The A3 and jr. have limitations because they lack the ports or outlets all together. The A3 pro has everything.
The Red Sea dosers won’t work unfortunately…. you can still use them, just will be separate from the Apex. You can plug it in to the energy bar but it’s only able to turn them off and on. Also, you don’t necessarily “need” the Dual Dos Reservoir… it’s just helpful to have if you’re out of town for extended time and need to see how much you have left to dose. I typically use the BRS mixing jug but whatever you have the dosing stuff in right now is fine.
If you’re not planning on going crazy I would just do the regular A3 and move essentials like the pump over to the energy bar. That way you can program alarms if anything goes wrong. Neptune unfortunately removed the salinity probe from their base models so if you wish to have that you’ll need to purchase it separately. Think they run about $100?
To answer your original question:
I would definitely invest in an ATO and some kind of reservoir or barrel for RO. It’s so nice to not have to top off all the time! Also would get at least two optical sensors to monitor the water levels in the sump. I like to have these so that if the water level gets too high or too low, an alarm will go off and notify me immediately. It’s pretty straight forward - “closed” means the water level has covered the sensor and “open” means the water is below it. You can program the ATO to work with these instead of a float valve but I would still recommend one anyway as backup. Neptune came out with the “liquid level sensor” a couple months back but I have only had problems with them, so I would recommend the optical sensors instead.
Sorry for the essay, but I hope this helped a bit!