I've been exploring this as well and have made a decision, so maybe you can benefit from some of my research. In my case, monitoring ph, orp, salinity, and temp are important to me. My initial plan was to buy a used Apex, then add-on whatever modules and probes I needed, plus add a WIFI bridge. The cheapest used setups you'll find are Apex Jr and Apex Lite, but Jr only monitors temp natively, so I'd need to add on 3 separate modules, 3 additional modules, and the WIFI bridge. For the Lite, it does temp and ph natively, so I'm looking at 2 modules, 2 probes, and the WIFI bridge. Assuming I buy all new modules and probes, I'm looking at about $600 and $700, respectively all together. The next option is looking for a used Apex Classic, since it'll do temp, ph, and orp natively. For this, I'd need 1 module, 1 probe, and the WIFI bridge. Prices for these have been all over the place, especially if they're including all probes. For something that has all probes and the add-on salinity module and probe, it seems to run around $600. A bundle without salinity would most likely bring that up even more, since those systems are going for around $500 and I'd either have to buy the module/probe or hunt for a used module and then buy a new probe.
The last option (and the one that I decided was the best fit for me) was to just bite the bullet and buy the new Apex that monitors these 4 parameters natively and includes all probes with WIFI integrated. This runs $800, but I just couldn't find enough cost savings with buying a used unit, since monitoring these 4 parameters are a necessity for me. Most used bundles that have everything I'd need are coming in $600+ and that's not even taking into consideration that you're using an add-on bridge for WIFI and you've have more modules to mount and wires to run.
If you're looking to monitor less, then it probably makes sense to buy preowned, because you definitely can find deals. In my case with the particular setup I'm looking for, it just didn't make sense financially to spend close to what a new unit would cost and have more components to deal with. The last tidbit that was an absolute end-all for me was the upcoming release of the Neptune Trident (automated alk, ca, and mag monitoring). The Trident won't work with any Apex systems other than the new 2016 Apex or an older Apex system that's been upgraded with a new Apex head unit ($400). I definitely want a Trident, so that kinda sealed it for me. Another thing to consider is that Neptune may use the same strategy going forward with releasing new components that only work with the new Apex, so that's something to think about as well. Hope this helps.
