I never actually won the game before my controller ran out of batteries and a got distracted with Lost Ark, but I got the controller going again and I'm gonna finish it. I'm up to the capitol, so probably not too far to go.
I leveled up my Int just enough to try a couple of staffs I have, but I didn't realize I also have to learn sorceries from a trainer first? I've never tried to use magic before.
So, regarding magic for a fighter, based on my experience, looking at my most commonly used spells...
My guy is a STR/Faith sorta paladin build with 50str, 27faith. There's not a lot of support sorceries in my bag.
I'd say, for a fighter, with minimum investment, 15 faith is enough to get you all the heals up to Great Heal, all the resist buffs, a damage buff, a regen buff and a few Beastal damage spells.
If you want to do good damage with spells it really requires a lot of stat investment. Like, there's a full moon spell that needs 70 INT. With 27 faith I can throw a boulder for about 350 damage which is only half what I do with a good weapon swing.
But, there are a lot of good weapons that require INT. My knowledge is dated but the cool kids used to talk about the magic katana a lot. It was, like... NE from the start near the Pot boy NPC, in a cave, iirc.
So, yeah, I dunno. You can't really go wrong, there's good options no matter how you do your stats. Most the FP might as well be spent on weapon arts and there's consumables you can use in place of buffs. But, early on, 10 faith to cast heal is like an extra 2 heal pots so it seemed like a good investment to get a bit of faith going.
I think I based my build around that gold halberd you get off the knight near the start of the game. That thing is awesome, but I see I've got another halberd at +19, presumably so I can put other weapon arts on it because I can cast Golden Vow now. It's really good in the mines where the miners resist a lot of physical but the holy damage destroys them.