well since your name is maegu i think you may be telling yourself something 
#i cant choose my main class
fickle narwhal
i think you should pick the one that you seem to gravitate more towards out of all of them, maybe DK since you listed that class first, but what i CAN tell you is that we get weapon exchange coupons fairly often AND there is a tag feature where you can change classes as much as you want
and if you just cant decide, pick lahn (thats my favorite) and then tag each class and retag when you get bored. this game is really fluid and we are getting a new class this summer (maybe sooner!) and it is very likely we will get a weapon exchange box with that release
you cannot change your characters class, you will need to make a new character (probably best to do a season for some back up gear etc)
but you can tag two characters, and they will share your best gear
the weapon coupons will swap the class, armor doesnt need to be exchanged (unless its DR to EVA) but that comes later on down the road