1. There're different kind of buffers in this game.
Melee buffers (buffers that aside from buffing, they hit at melee range): Bladedancer (Dualswords) and Swordsinger (Sword/Blunt/Dagger + Shield)
These buffers offer short duration buffs that are highly useful, Bladedancer being more on the offense buffs and Swordsinger being, mostly, defensive buffs, among others. Bladedancer and Swordsingers are also highly needed (SWS a bit more than BD), so generally you'll find parties easily with those.
Robe buffers (buffers that actually casts the buffs and wear robe gear): Prophet (Great buffer for fighters, archers), Shillien Elder (excellent buffer for mages and helpful for fighters thanks to Vampiric Rage, which lets you recover HP as you hit, only for melee) and Elven Elder (nice tank buffer/healer, really useful buffs later on, see Clarity for example, which decreases MP costs)
You'll find that most of these buffers, except for Elven Elder sometimes, are played as boxes. Note that Shillien Elder and Elven Elder are the only buffers that can recover MP. If you go one of these three, you'll find parties but sometimes they won't need you since they have a boxed one. This specially goes for Prophets, which are the most used box imo.
Whateveryouwanttocallit buffers (buffers that can wear any kind of gear, those are the two Orc buffers): Warcryer (have most of the physical buffs with a few defensive and magical buffs) and Overlord (most of the magical and defensive buffs, also is a character used to bring different kind of diseases to the enemy, not only in the game.
These buffers can offer a variety of buffs, they don't focus on X kind of buff (just like Shillien Elder is focused on mages). They can even go leveling at melee range.
Warcyer is welcomed in almost every party, Overlord is a weird character, may be the least used buffer, it doesn't mean it's useless. In fact it's highly useful on Clan vs Clan, since the cancer they can puke can reach Mars (AoE CC, tons of it)
I suggest you go Swordsinger > Bladedancer > Warcyer
2. It's always useful to start as a spoiler. As simple as that, you'll earn more money at the start if you are new here.
3. No subclasses on Classic 2.0
4. A lot of guilds are recruiting newbies, check the forum section or go to discord/in game chat
5. A lot