Once you get to the shipwreck camp you'll be able to buy a better bow.
I played a ranger on nightmare difficulty, and didn't get around to learning Splitting Arrows until the end of Sentinel Island. I assigned that skill to the long-unused melee attack slot, and never missed it (early in the game I had sometimes fallen back to unarmed combat, before switching to a more run/jump combat style and adding Stun Arrows to supplement Poison Arrows). After that, most opponents in melee range were easily taken out by a point blank splitting arrow.
I usually don't bother with temporary boost spells, but after buying a bow on level 40 with a Way of the Ranger skill bonus, I tried it out, and put a few skill points into it myself (and later maxed it). It is quite helpful, especially using before heading into range of a group of opponents or a boss fight.
You don't need strength, except a bit early in the game for the healing rate boost. IMO you don't need intelligence except for resistance to Stun Arrows (that's pretty much all I had trouble with, anyway, not boosting intelligence or spirit); Mana Efficiency and Increased Mana enchantments (once you get armour with 2 slots; I generally went with Life Line first) can take care of your mana requirements.
FWIW I stopped distributing stat points after level 15, to keep the game challenging. I described my playthrough in the topic
Who's playing DKS on nightmare difficulty ?, along with a couple other people, so you would have to be careful of spoilers.
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