I’ve been looking forward to LA’s western release for a good while now. When the game got delayed in the autumn i decided to give it a try in RU.
From my point of view i usually prefer range DPS glasses, usually wizards/mages in games. I gave summoner a go, and bard as they were both the mage glasses going to be available at launch. I enjoyed both, only issue with bard is it is definately slower to get through content (but not painfully slow and i will likely play one to some lvl).
I then had a go at gunslinger as i thought guns that must be ranged? well that turned out not to be the case, it’s very much close/medium and a bit of long range. I also didn’t really get on with the weapon switching, i think that’s probably something to play once you learn the game.
So next i tried sharpshooter and yep that is a true ranged DPS class so if that’s what you like it will suit you well.
As for needed classes, generally from what i’ve seen for end game content people like 1 support + 3 others per group, so that’s one bard/paladin + any other 3. There are some that synergise better than others in the DPS 3 but generally they all work. As it’s more about player skill than the glass or gear for most content. There will obviously at some point be the artist/painter whatever it will be called who might be a 3rd option for the support.
I would just have a go with a few classes and not worry about it till you decide which you like. Alts will all be useful anyway just don’t invest too much gear in any class until you decide.
So much of lost ark unlike most western MMOs use “account” progression, which is a very Korean MMO thing. So alot of the time you invest will progress your account and any whatever becomes your main eventually anyway. eg islands, estate etc.
Her in the west we’re very much trained into “Main” mentality, and yes in LA eventually you will want to concentrate building a main, but there is way more ways to progress your account in general than western MMOs. So relax a bit and enjoy 
Edit i should also say when sorceress was announced and summoner (which is what i was going to start with) got removed i went back and tried sorceress a go and that will be the class i concentrate on as that’s what i usually migrate too anyway and it was great fun.