So what's the max hero level in this game? Is there a max level at all?

I read someone saying he was over 300.

The limit is only technical, as in the game will crash trying to handle too large numbers (damage, hp etc.) eventually. In theory, you can reach level 1000 if you do not invest in skills or stats. (you still get some extra hit points on level up even without using stat&skill points if I remember correctly)