I treat enchanted arms and armor slightly differently from core rules. This affects not only the enchanted items, but a few other rules as well.
As in core rules, weapons must be masterwork in order to be enchanted. Unlike core rules, I allow ‘+0’ enchanted weapons. Such weapons must have a special ability, however. That is, you cannot have a ‘+0 longsword’, but you may have a ‘+0 flaming longsword’.
This interferes with the changes to damage resistance, though. DR #/magic, in core rules, means that any enhancement bonus on a weapon allows the weapon to ignore DR. Allowing +0 enhancement bonus effectively means that such weapons can ignore DR, as written. Since I don’t really like that, and I’ve never cared for the binary nature of damage resistance, I use a different mechanism.
DR/magic gets reduced by 5 for each +1 enhancement bonus on the weapon used. A creature with DR 10/magic gets full DR against all mundane weapons and magic weapons with +0 enhancement bonus. Against weapons with +1 enhancement bonus the DR reduces damage by 5 points, and against weapons with +2 or better enhancement bonus the DR is defeated altogether.
Like weapons, armor must be masterwork before it may be enchanted, and +0 enhancement bonuses are allowed as long as the armor has at least one special ability.