I don't think you will need pH buffer because the kalk should maintain good buffering. Add it at night via slow drip (drop a second or slower) and that should maintain your Ca and Alk to get you started. But first keep an eye on your Ca and Alk levels that way you know if either your keeping up with your tank demand or not dosing enough. Saturated kalk has a pH of 12 so that's the reason for the slow drip, you don't want to overshoot your tank.
I think you would enjoy MH but 4x55w PC's will be okay. Keep in mind bulb replacement cost. A single 175w 10k bulb might be nice over that tank and just as effective on keeping corals ect. Plus you get the MH "shimmer" which is priceless. But anyways go for what you want I'm just throwing suggestions out there. But once you use MH you never go back
