First char: Sair

Probably gonna want a DK in your party if you're a beginner and a Corsair because if you want to level the char yourself you won't have too much INT

TBH unless you want to remake your Corsair after your first unwashed one, have a really bad time levelling up, or just forget about endgame bosses, you should pick one of the many other jobs that don't require washing instead. You can't have your cake and eat it too sadly no matter how much the staff want to bullshit about how 'unnecessary' HP wash is to sell their server. It is unnecessary, just not if you want to do all bosses as you say

Best first chars are mages especially Bishop, and if you want to boss with a mage the only viable one really is Bishop/Cleric

I personally went Warrior first because mages bore the hell out of me, any Warrior can be played unwashed but mainly Hero or Paladin since you will need to wash as Dark Knight to safely do full damage in all bosses

You can also play Bandit this class can be played totally unwashed and be fine, this is a decent pick buut 2nd job is not the easiest grind. Not bad for bossing

Last first char choice is Buccaneer, the other pirate so you might enjoy this one. Melee class though, still requires washing but not nearly as much as Sair (you can level up with this and not suck during early, mid or endgame0 and not considered the strongest but awesome for parties due to amazing team buffs