I have killed many of the handheld dip in units ;/ They are pretty much toast if they dry out and keeping them moist is a royal pain. The remote probe unit I have now is great. The probe is not cheap. About as much as the whole handheld unit but it is far superior in quality. Does not need to be kept sitting in solution although it is easy to do if you wish. IE it fits nicely through the top of the solution bottle.
Personally for CP use you really only need to test your RO system once or twice a year if you do a good job changing the filters and maintaining the system. Probably easier to just find a decent aquarium shop or pool shop somewhere and paying them the few bucks to do a quick test for TDS.
These days a good typical 3stage RO unit is easy to come by and they all use standard housings. Most companies selling them construct them from parts made by central manufacturers anyway and just stick their own label on them. As long as it's a TFC membrane, what is important is to check the quality of the prefilters.. Solid Carbon block as opposed to granular and a separate 1micron (or less) sediment filter. Unit like that and you will have plenty pure enough water for all cps..