That PH monitor would work fine. As far as RO water I have a 100GPD unit, It was not cheap, but the water is pure after it comes out, all I do is put in a buffer and let a powerheard run in the bucket until the PH reaches 7.0, then its good to go. I dont test anything else but PH and like I mentioned before I have a PH controller that shuts down my co2 when the PH hits too low a level.
You can buy RO units that make less water per day, that are cheaper then the one I'm using, but I needed the large amounts of water for all my tanks. If you have 2 small tanks then look for a 25GPD unit, and buy a 35 gallon plastic trash can with a lid, drill a small hole in the lid, and insert the RO water tube into the lid, hook it up and turn it on. At 75f an RO unit rated at 25GPD should make 25 gallons
RO is not necessary, but my water isnt the greatest. You local LFS can test your water for you, ask them what you would need, or if the tap is fine.
Testing should be done until you get the hang of your tanks, I tested Nitrates, Phosphate, and GH, KH, and PH. I also got a little excited and tracked Iron for a while, but gave up after I decided to put EcoComplete substrate in my tanks. I dose Phosphate weekly, but no longer test it, since I have gotten the dosing for my tank pretty much down pat. I no longer test GH/KH but I have RO water, and add a buffer based on a PH goal of 7.0 and I have all softwater fish.