I have the KH Guardian, and will be upgrading the High Precision Nozzle and KH Enhance sensor soon. This is a good deal as I bought mine (monitor version which doesn't dose Alk only tests) for $420 from Coralvue Outlet ebay. This is for the controller which also doses Alk for you. I find this is a very solid piece of equipment, but can also accept any pH probe although they have a new one being released to. Either way, would need to calibrate the probe about once every month or other month.
For the upgrade if you DIY on an older unit, the high precision nozzle is fairly easy to install, but the KH Enhance sensor requires soldering like 2 pins or something like that. So if you aren't comfortable doing that, better to get this deal.
Obviously Trident, KH Director, and Alkatronic are great choices too, but next year seeing the GHL Ion Director, Xepta aBex/autobalance, and Mastertronic is going to make things really exciting. The Ion Director and Mastertronic will be able to run on it's own, so I'll probably pair one of those to my KH Guardian.