If you are spending that kind of cash...and now you have the Apex Jr, you definitely should go for the ATK. It is only $10 more but comes with a pump already (although some have had issues with the pump, most are fine). It has multiple levels of redundancy to save overflows. It can be used whether you have an Apex or not. The advantage of having an Apex is that you can set up alarms and control it through the Apex. If you add a Break Out Box (BOB), then you can even add level sensors to tell you when your ATO container is getting low. Much like every ATO, there are horror stories out there, but that would be the way I would go if I still had an Apex
With that said, you can also do one yourself with just a BOB, a pump and a few good float switches. You could build a good one for less then $100. I would use 3 floats. One that when the water gets low it turns on the ATO pump, the second will shut off the pump when the water gets high enough, and the third would be above that to set an alarm if the water gets too high. If you DIY everything, you can probably do it for less then $50. The Apex BOB can be built vs buying it. Heck, I am pretty sure I can 3D print you the brackets even.