It depends on what kind of corals you intend to keep. If you're going to keep nems and softies, I wouldn't bother with calc/alk/mag but you'll probably want to test nitrates and phosphates. Iodine is important to soft corals but if you're doing regular water changes, I wouldn't even bother testing it, unless you think there's a problem. For testing nitrates and phosphates, Salifreit, Hanna colorimeters, Nyos or Redsea are all fine. For testing calc/alk/mag (if you're keeping Stoney corals), I've used all of the above except Nyos. I honestly prefer Salifreit for those. They're pretty cheap and easy to use. I have Hanna colorometers for alk/calc/phosphates and I do use them occasionally to cross check things, but I'm not a big fan. They're just as accurate, but adding the little packet of powder to the vial annoys the hell outta me. The Nyos kit for phosphate could be better. It's a bit hard to read at low range (under .1ppm). Oh yeah, if you do go for the colorometer for phosphates, get the low range PHOSPHORUS meter. It's a lot more accurate but you do have to convert phosphorus to phosphates (it's not that hard).
Oh yes, if you do keep SPS coral at some point, you might consider testing for potassium, especially if you start carbon dosing.
Finally, if you get really serious about wanting to know what's in your water, you can look into ICP water tests which requires you to send water samples to a lab. They test for all kinds of things and there are several companies doing it thought Triton was the first I heard about. They give you test results and let you know if anything is low or high. I've yet to get one done (they're around $50) but may once my recent build matures a bit.
I know it probably seems overwhelming, but just start out testing nitrate and phosphates and try your hand at keeping some softies and LPS. Do regular water changes and you'll be fine. Softies aren't going to use any calc/alk/mag and you'd have to have a lot of LPS to need to worry about calc/alk.
Just my 2¢.