I've been challenging me and kind of getting places too dangerous - it's fun. I'm now level 6 (i think) and i want to join Mercs. What would be good level to go to the test? I think level 6 is way too early, especially i'm playing ranged build?

You have to be level five to start the quests, so the game sets a pretty clear border there, which you could pass by now.
Otherwise there is much you can do before joining a faction, it is easy to get to level ten beforehand, if you want to.

But I'd say you'll do just fine, joining the mercenaries now.

And if you haven't already been to the farm, be sure to get Lares to go with you; it makes things a lot easier...

CAVEAT: make sure you've already been with Lares to see the water mages though, otherwise the quest will glitch out.