TLDR: Berserker is a hard class to do well, she can try but she needs to blockcancel or gtfo so...if she can do the timings its alright there is more skill then buttons to it. If she cant its horrible.
Priest is nice, tanky as hell due to heal and not that hard to play okay as long as she can aim with the mouse its fine.
Gunner is op at leveling, your gonna have a hard time not playing it. If you want melee though, reaper is okay (mediocore dps, but balance changes) and if you like magic stuff it very much looks like your perfect class.
If you go for "best" classes, zerker does best dps along with slayer which is the best melee class maybe if u like funny swords.
Warrior is...i dunno people call it hard cuz it has so many skills but it really isnt hard and..most played class. Also, a bazooka is more realistic and less silly then the never ending 2x1h sword fantasy crap (got a warrior to 50ies, if i imagine its daggers and im a rogue its okay:D).
Wizard is much qq and much micro, people still play it so why wouldnt u if u like them?
And yeah, its great fun you get double xp this month lots of free stuff and its as fair as online games can get moneywise (you know...people bought progress before game devs sold it and if u nolife it you can be elite without real money now).
I just started a while ago and i regret not playing it right after beta, maybe cuz im from the eu and i got the sucky version of tera before i stumbled upon na.