There are only few melee weapons requiring intelligence that I've come across. I think they were maces or such. I only saw a few armors requiring intelligence, but it wasn't that much. 40 or so points at most, and that was with a high level requirement too. Even if you distribute your stat points across all your abilities, you should still be able to wear specialty armors, though strength is probably the most important and will give you the most bang for your buck when it comes to armor. I kept some points in reserve and only used them when I came across cool armor I wanted to wear. Then I'd bump the strength until I could wear it. I did not, however, waste strength points to be able to use the biggest shields. That would have cost way too much.
You get the same XP regardless of who kills the enemies, afaik. I've even gotten XP from enemies killing themselves. So charm away!
You should spend some more points on strength and vitality, so you can wear some better armor. You're a dragon slayer/knight, not a pure frail wizard. I guess it's possible to go through the game with low vitality and strength, but I certainly didn't do it - and I played a wizard/archer type mostly.
At the end of the game, I had about 400 HP and 350 Mana, when wearing all my equipment. All my jewelry had regeneration enchantments on them too and I was almost indestructible

Any more is almost a waste, but of course playstyles differ

Fireball pretty much sucks. Not because of its low damage (it does get better) but because of the long cool down. WAY too long. Instead, invest in a skill like Magic Missile. You can spam that way more in the time it takes for fireball to cool down and it will do incredible amounts of damage. If you want a fireball-like skill, wait for later and invest in Explosive Arrows. It's MUCH better than fireball.
Destruction is an ok skill later on, but I would suggest mana efficiency before you use destruction.
Destruction gives an initial boost of 18% but you have to spend two more skill points to get to 30%. So hold off on more than one point, until you have points to waste. Mana efficiency makes you into a spell casting machine!
If I remember correctly, my skills at the end of the game were, Magic Missile (12), Fireball (1), Charm (5), Mana Efficiency (5), Destruction (3 or 4), Summon Demon (2), Explosive Arrows (10 or 11), Lockpick (3), Mindread (1). I also had the magic armor skill (forget its name) at 5. Maybe a few more, but nothing special.
If you want to play using ONLY mage/priest skills, that's okay - just be aware how weak fireball really is because of the cooldown. You get to send it off maybe once or twice in a long fight. Explosive arrows can be fired every 5 seconds.