I would recommend a different system. 3 stage RO DI can work, but you will prematurely wear your RO membrane, and you will go through DI resin much faster. Both of these add of rather quickly, and are the most expensive parts to replace in a system. It is much better to have a 4 or better yet 5 stage system, that includes some additional prefiltering before the water hits your membrane. These prefilters are about 10% of the cost of a new membrane, so it is worth the upfront cost to save some money in the long run.
That being said, a really reliable and good system can be had for not much more than the system you are looking at.
BRS sells a great 5 stage system for $199, or $179 if you get in on a
Group Buy. The 5 stage plus I linked you to also includes a PSI gauge (something you really do need to know when it is time to replace the prefilters, especially the sediment filter) and it also includes a dual TDS meter, so you can see the TDS in to the DI canister (to know how your RO membrane is doing) and the TDS out of the DI canister (so you know how good the water is, and when it is time to replace the resin).
And yes, you do not have to add anything else to the water, except salt mix for water changes. For top off, it is ready to go - no dechlor any any other chemicals required.