First and foremost, you want to check your salinity:
http://www.marineaquariumsa.com/showthread.php?t=41231
Get a decent refractometer.. Last I checked, the D-D ones around R900 from Jungle Aquatics. Alternatively, find a conductivity meter, more info in the link above.
Secondly, you want to measure your pH, to be honest, I would not waste money on a kit here, rather go for a pH pen or monitor. Good brands = Milwaukee & Hanna are good brands.. In fact there is a good quality second hand one for sale here on MASA:
http://www.marineaquariumsa.com/showthread.php?t=41864
Thirdly, you want to check your ammonia. Once your tank has cycled, you will very seldom use this kit again, so perhaps better to have a reputable LFS measure for you. Alternatively, get a seachem ammonia alert.
Next is nitrite, same as above in terms of post cycle.
If you want to get a kit, get the Seachem one, as it measures nitrate and nitrite. Nitrate you will keep using.
After nitrate, you will need to start looking again at the inorganic chemistry. Alkalinity is vital. Here most kits are pretty acceptable, as long as you stay away from the cheap nasties. Red Sea, Salifert, Seachem are all fine. If you want a really top end one, source a La Motte or buy one of those Hanna checkers from Reef Aquatics.
Calcium and magnesium are next, although you only really need to "worry" about them when you start adding corals with a hard skeleton.
Lastly, back to organics, would be phosphate. DO NOT WASTE YOUR MONEY on any kit, other than the D-D high sensitivity one. Alternatively, get the Hanna Checkers, Reef Aquatics Sells them. Another slightly more affordable option is the Milwaukee checker, the same accuracy as the Hanna checker, for a few Hundred less.
Hereafter, you might want to get into the more in depth kits like borate alk, Strontium, Iodine, etc,