The game codes are for the recruitment campaign. You get free stuff and so does the person that give you the code, nothing really crazy just some extra goodies.

The expansion is something you wont need for a little bit of time, you have to complete the storyline up to version 3.5 to even enter any of the expansions content, so if your not sure about the game there really isnt a huge reason to have it til your are well into the level 50 content. Other than that it plays similarly to most theme park style MMO's ( WoW being the closest cousin) although FFXIV uses a slower GCD but more combo centric skill strings. Pugilist ( or monk) is really about stringing combos and being aware of the bonus in your skills that comes from proper placement in relation to the mob.

As a new player though just make sure your always up to date on the MSQ ( main storyline quests) as they are the primary gate for almost all the content in the game. Don't worry the story is actually very good once you get past some of the standard fetch quests.

Good luck though!