It's a bit, errr a lot, overkill. It also doesn't have the range you need for certain things that are important such as calcium. The only thing I really test for any more on a regular basis is KH/alkalinity, and you will probably get to that point before long.
Ammonia, unless something is terribbly wrong you're not going to have it in an established tank.
Nitrites, pretty pointless to test for in saltwater. The only time worth testing for it is to make sure your cycle has been completed in full.
Nitrates, if you set up a proper system you'll get tired of testing and getting 0 results over and over again.
Phosphates, getting a seperate Hanna kit for this isn't a horrible idea. However, the amount of algae you have and how iften you have to clean your glass is usually the best way to know if you have elevated phosphates. Plus you can buy a pretty good amount of phosphate remover for the price of a hanna colorimeter. Either way you're probably going to need to buy phosphate remover and a reactor...
Magnesium, after youy've established your systems needs you'll only need to test every now and then to adjust your dosage.
Calcium, see above, except if you're using a 2 part or a calcium reactor you can usually just adjust according to your KH levels.
For test kits I suggest API for alkalinity, calcium, ammonia, and nitrates. For magnesium, Elos or Salifert. For phosphates I say no test or the Hanna colorimeter if you really must know your readings. The rest you can skit IMO.
Now take all that money you saved and spend it on lighting, a protein skimmer, and lots of flow. Those are places where splurging and spending top dollar is often worth it
