Well, you're going to have to start monitoring alk/ca relatively quickly. Especially since you'll wanting to start adding a bunch of corals, but take it slow.
IMO I would rather dose 2 part then mix kalk with my ATO. I'm assuming it would be more stable and consistent, which is what matters in the end (this is just my opinion).
I use 2-part from BRS. 1 gallon of 2 part (alk/ca) lasts me about a month and a half (about 70ml of each per day). I have a calcium reactor that I will be setting up in the next couple months, hoping it will save costs over the long term as well as increase stability and consistency.
https://www.bulkreefsupply.com/brs-2-part-calcium-alkalinity-kit-pre.html
If you do start researching dosers, just know that the price fluctuates quite a bit. The higher priced dosers can dose accurately to .1ml increments (useful for dosing color programs) while cheaper dosers (such as jebao) only dose down to 1ml increments.
You could get a cheap jebao 2 head doser for 75$, which would be fine for alk/ca. It's just easier, more stable and convenient for me (just be sure to take the appropriate measures and try to plan for equipment malfunction).