How do I level faster? Quests or just doing straight DGs?
just to the yellow quest which are the main story line, you can also complete the green ones on the way since those reward you with some materials and accesories, and some good equipment to help you on your journey
What is the level capp?
How do I get costumes?
buying bits for the gacha, or buy them from the bm, can be crafted too but those are quite limited in stats
How do I farm for gold?
once you get to the last level you can do the dailies-puri zone- and pgs zone, or buy bits and sell them for in-game money, selling you constumes, materials also helps too.