The PvP is very competitive although maybe a bit unbalanced. This game is pretty gear dependent, and the difference between good gear and great gear is pretty large. There's large scale wars every day for control over different areas of the game (which provides the victorious Guilds with gold and another sort of prize which you can determine the value of yourself later on). There's Guild vs Guild elimination tournaments every weekend among top ranked Guilds. There are nearly hourly Arena sign-ups and three different subcategories of Arenas. And if you want to challenge somebody one on one, you are allowed to do so outside of dungeons, designated PvP battlefields, and non-PvP areas by offering a duel request.
As for social, finding a Guild is usually good for that. If you are ever feeling lonely, you can always visit Aven on channel 1; it's packed.
All classes are useful in dungeons. Tanks and healers are necessary, and all sorts of DPS are needed since different bosses have different weaknesses. If you strike at these weaknesses (measured by a "Break Point Gauge") and deplete the boss's BP Gauge, it will drop more items of higher rarity.
All classes can be good; mostly the gear and experience determines effectiveness. Hunter is good at single target dps and has the ability to block healing for a short time (very dangerous for healers in PvP). Engineer is the best early game farming class starting from level 30 being able to spam its AoE skill Oil Bomb very, very quickly. As you get up to higher levels, different farming classes catch up to the effectiveness of Engineer and perhaps surpass it, but it remains one of the easiest classes to farm with. Ranger is a top-notch PvP class with excellent control over the flow of battle. It also has high damage output in PvE, but is somewhat lacking with AoE. Melee dps tend to hit harder than Range dps, but they need to get up close and personal. Magic dps tend to hit [significantly] harder than anything else, but they lack bulk of either Range dps or Melee dps (especially since many Melee dps classes are viable with Shield usage). Templar has great survival ability, but tends to have trouble holding aggro in dungeons. As such, people prefer Knight or Warrior for tanking dungeons and Templars for PvP (since they can do high burst damage easily).
Depends on how you get gold. If you are good at merching items, it's easy. If you prefer to grind, it's decent, but takes time and patience. If you buy AP, it's pretty easy to get lots of gold fast. This isn't a pay to win game, though. It's easier if you pay, but regular grinding at later levels lets you generate all the gold you need to buy top gear at a reasonable pace; you just have to be a bit patient.
Whether you should spend gold on a skill or not is up to you. I suggest not leveling Prevention for healer classes, though. Provoke and Cure are must-level skills if you are tanking or healing respectively.
My best advice is to try out different classes and see what's best for you.
The play style of physical dps classes will change as you level. At first, it is best to try and spam skills while allowing an autoattack in between skill usages. At later levels, notably level 65, it can be seen that Atk Speed increases in importance and usefulness, so it changes to mainly autoattacking with key skill usage or buffing in between. However, a powerful skill spamming build is also totally viable, especially if you are using AoE to clear mobs faster.