You will get a shit ton of different answers. My thought is you can't really have too much, but you also don't need everything.
I'll try to answer point by point. Note this is my opinion. You will find your own in good time.
1) No rule here. Most are fine with 2 kites. I tend to think 3 is a good number then add on once the extreme conditions come around. Note that there will be one size you use more than others and it will be location dependent. Make sure you get that kite and not the sizes around it.
2) No rule again. My thought is I need 1 TT, 1 Foil and a few surfboards. I like to ride waves the most, then foil and then TT occasionally. You have to figure out your own priorities here and then decide.
3) No. Kites today are great. For instance, I ride the Naish Slash. It is a super forgiving kite for waves and it is stable in the air. Those qualities are something you are looking for in the beginning as well. If you go towards a kite like the Pivot because you prioritize jumping on TT, then you will still be fine. It is a bit more lively and explosive when jumping, but the kite is also being used by the Red Bull king of the air champion.
4) If you get more gear you will have more leverage to negotiate. As you progress in the sports you might find you are fine with other buying strategies. Many people buy used. I think in the beginning it will be a bit harder for you to judge the condition of the kite so I'd venture toward checking out shops.
5) Get the brand you like. Personally I think Core riders are like the BMW drivers of the kiting world. I am kidding.
6) It depends. Both good boards and good kites will keep you happy for a long time. I would rather buy a used TT than a used kite. If you get a used TT you can buy a super big board until you get your first rotations nailed down.
7) Kite size is dependent on locations and average winds there. No one should give advise on this until they know the location.
9) Be careful of buying used kites. People will sell a lot of shit. Most are fine and most are honest, but there are a few traps here.
10) More important to pick the right airlines. Most here has "golf-bags" and they work, but you might not need it anymore.
11) Depends on where you are traveling to. I would just try to get good quality gear. Nothing more annoying then to sit and watch you fellow kiters rip while you have a kite to fix.
12) Probably. But again location dependent.
13) Real is great. Ebay be careful. If you are looking for packages give Real a call and they will hook you up.