Lol, advertising your friend code eh? ^^' But he is right, the recruit a friend rewards are quite nice. The biggest perk of it is probably the friendship circlet. However, for your first class, it probably won't be as beneficial as it will be once you get your bearings. You'll learn as you go what you should do and what you shouldn't. If you do get the rewards, as a heads up, use the silver chocobo feathers on the ironworks gear for your lvl 50. Yes, you won't be able to use it for a while, but if you get the early stuff, it will become irrelevant REAL quick for you. At the very least, don't get the lower level weapon (as once you pass that level, you really won't be needing it anymore once you find a better one unless you REALLY want to glamour it). You'll even get to the point of not needing the ironworks gear, however, when you hit 50, you won't have to grind for poetics for those which will give you a good boost right when you hit lvl 50. If you have better gear than what ironworks can provide you by the time you hit 50 the first time, i'd be VERY surprised.

Now in terms of your actual questions.
1. Yes, gathering is great for getting more gil. However, you are not at a point where you need to do that for gil in all honestly. Main story quests (MSQ) will give you PLENTY of gil. Just don't waste it on stuff you don't need (like I did). Be very careful when looking at the market board as well if you're like me and happen to have the mindset of a fish (oooohhh, look, something shiny!!). If you stay on top of story mode as you go, it will usually provide you with equipment that's just the right level for you. If you like to be slightly ahead of the curve though (or really like doing all side quests before moving on, then yea, you may want to look for where to get higher lvl gear prior to the MSQ gear. Vendor's are DEFINITELY where you should go to get upgraded gear. Market Boards are hardly ever at market value for the items being sold. It's a convenience things. Time vs. Money kinda deal. You could go to a vendor in the middle of no-where or spend 10 minutes collecting what you need out in the field, or you can pay 10x the normal price. Seems expensive, but at the same time, some items are like, 15 gil a piece, and you'll see it on the market board for 50 a piece. You buy 20 and you really only spent 1000 gil. But adds up quickly if you aren't paying attention.

2. That is to buy the actual game/download. You get a free 30 day trial when you buy it. You need to pay subscription from that point onwards. And this is not to include heavensward expansion. But play ARR first and see how much you like it (I totally ended up buying HW when I hit lvl 50).

3. Yes. The nice thing about this game is, you can change classes with 1 character very easily and if you want, level all of them on your own time. Many of them share equipment for a while as well, so it's also nice for saving yourself some money instead of getting rid of it early, then end up with another class that uses it and you have to go buy new armor that you already had at one point.