If you're just dosing Ca and Alk, I would get the tunze auto top off system and use kalkwasser with it. I used to to do the 2 part dosing, but since December when I got an ATO and started using Kalk, I would never go back! Unless my demand exceed the saturation of Kalk. No more manually daily dosing, no more manually topping off. I fill my ATO reservoir like once every 10 days and good to go!
But if you want to do 2 part, I recommend BRS.
+1 on the ATO + Kalk though! Kills 2 birds with one stone. Not very expensive at all. EASY to set up and use. Dam near plug and play. Just need to figure out out much Kalk per gallon of ATO to keep everything stable!