1: Event brings you to Serpentium, which also leads you to Doom Aporia and Atma'Ram. Doom Aporia you don't need to worry about but Atma'Ram is good since you get the best EXP farming spot currently (Magma Breath), a daily entry dungeon for new magic stones (Aqua Whistle), and giant boss dungeon for the new BIS accessories (Spirit Lord's Temple)
for Monaterra, you'd really only do it to get better Harconium. You can use the ones the game gives you and be fine. for Serpentium, you'd do dailies to work on the Exascale armor and get circuits for it
2: game kinda expects you to learn quite a bit by the time you get 3rd advancement/master class. if you're curious on how to properly play/use your class then asking is your best option
3: for a 5/5 you'd need around 1b or so which depends how much you're willing to farm for ED. do your weekly SD's and weekly missions, and depending on the ones you do, those can get you about 200m each week. event character has extra entries so that's an additional 100m.
for accessories, that will cost more but the elrios guardian knight accessories are fine. anniversary tends to give a free IB set, nothing too recent, but can still be useful. KR got Bloody Nightmare so we'll probably get it too for ours but we'll see in like 2 and a half months
