I'm a big fan of the SeaChem test kits. They give great results and aren't too expensive. The things I test for are as follows...
Calcium and Alkalinity - Since this is the most important thing to test for in a reef, I do it twice a week.
pH - Since I'm testing for alkalinity so often, pH is less important. I only test once a week.
Nitrate, Phosphate - Once a month at most. I haven't detected nitrates or phosphates in months.
I'd recommend not using the dKH
buffer unless you know you have alkalinity problems. A buffer is a pretty bad way to fix pH problems. The only time I would use one is if I had low alkalinity AND low pH.