don't ever let your HP make you feel like you can't melee. I can keep up with the beefier DRKs in my level, and in some cases, outdamage those higher than me (DoT-speaking).
This is how I considered it when I chose dark knight:
Ok, I don't have a lot of HP. What *do* I have? Boat-loads of INT, higher DEX and AGI than everyone except a Mithra, and I as such, I can load up on STR as much as I want.
My boat-loads of INT mean I can absorb-xxx all day long, buff myself while debuffing the mob, and waylay the mob making it easier for everyone to kill.
My good DEX and AGI means that a) I don't necessarily *need* a crapton of +acc gear (read: PCC, snipers), and b) while when I *do* get hit, sure it hurts, I *don't* really get hit that often.
Loading up on STR? 'nuff said.
Don't ever discourage yourself from something that looks interesting because "meh, I don't have much HP". If that was the case all the time, you wouldn't see any Galkamages