A few months of necromancy here but this is under discussion again, and reviewing this thread brings up a few points.
First though: Why would you ever spend money on a magic weapon at character creation? Why, so you can kill monsters that can only be killed by magic weapons. *shrug*
A few other notes:
Diffuse creatures can be damaged by area of effect
damage of any origin, not "spells" specifically - alchemical fire certainly suits that, and I recommend having at least a little around. You can make do with just one I suppose, if you can get creative about pushing them into the flaming hex, but I'd recommend a party start with at least two.
Affect Spirits would work IMO to allow a weapon to do full damage, although it's not a quick and easy cast.
"Magic weapon" is not formally defined, and is an interesting question. I definitely count weapons that are under the effect of a Flaming Weapon or Flaming Missiles spell (and Lightning Weapon and so forth) - it's not a permanent effect, but regular old enchantments don't have to be permanent either.
Those are some straightforward "weapon-y" effects so the questionable part is that it's not formally enchanted (I fall on the side of the spell counting as this means parties can plan for them a little better).
I count an Affect Spirits weapon as sufficient to kill them dead like the flaming weapon. It's a weapon targeting spell and it's specifically for killing spirits so I feel very solid on that one.
Is a weapon with any random magical effect on it a "magic weapon"?
That's a more interesting question and I'm going to go start a thread about it.
Asked the question
over in the DFRPG forum