1- It's highly recommended, but play it a bit first, get to Temple Station, then if you feel like you like it enough, do the subscribing
(Temple Station is where you can enter Templar Base and then the Stonehenge within, which is the subscriber only area).
2- Most prefer Elite (I'm one of these people!). It's way more fun IMO, more challenging and rewarding as well! Norm is a walk in the park compare to Elite. There's a but tho: Elite is kind of time consuming because of the special monsters, so let's say, and area that took 5-10 minutes on normal mode, in Elite it's gonna take 10-30 minutes depending on the mobs that were spawned, your gear, skills, if you die (when you die in Elite you're sent back to the station, can't spawn at the start of the area). After you get used to Elite you can handle it pretty easy too. I managed to lvl up to 31 without any problems in 3 days in Elite, in Stonehenge.
3- Not really. Just go with your favorite class and have fun with it. They're pretty much all amazing end-game, but you got to like your class, or else you're gonna wish you picked one of the other classes. The all-time favorite is MM, they can destroy anything from afar and sort of stay out of harms way, not mention it's a break of the classic hack-and-slash that most RPGs are based on. If you're looking for high survivability tho, Guardian is the class. Well built and skilled Guardians can own mobs 10 lvls higher than them (I got a friend who soloed his way through the game at lvl 32, but he was godly, he had the best sword (Hikida's Lamentation) and the best armors (Sol's Invictus and Jugalator's), and was using them. They are fun too, you can choose between using a sword or a pistol with a shield (feels and looks like those S.W.A.T. members that go in first when they're entering a build, with the iron shield in front of them and all, looks pretty cool).
Sorry for extending myself, just wanted to be as clear as possible!