Here are my thoughts:
1, The Carbon doser is a waste of money*. I've had my reactor up for a year now with a cheap old-school needle valve ($50), and never had to touch it - *this is only true if you're using the now so popular constant flow method, other wise it would be a PITA.
2, If you decide to go with the constant flow & regulate the pH in the chamber with a controller, then
a, You don't need a peristaltic pump, it can be done via a seperate cheap pump, or from the manifold (however it is recommended to add a filter inline, most people use fuel filters) - this gives pretty stable results, but needs some attention every 2-3 months. I'm not saying that a peri pump isn't better, I'm just saying that the results without it are adequate.
b, You don't want an oversized reactor, as it will need a lot of CO2 to melt the media & will lower your pH in the tank a lot (remember we're still talking about the constant flow method) & it will be hard to dial in the bubble count to keep the solenoid clicking all day. You don't want to melt the coral in your tank as well, do you?
If you want to go old-school and use the dripping method, then IMO the Carbon Doser and a Peripump (Kamoer being the cheapest, but it's chingchong at the end of the day, don't forget) is a must. You can get away with a bigger reactor with this method though.