According to the Pathfinder SRD site,
These bonuses can be added to the weapon, stacking with existing weapon bonuses to a maximum of +5
So you can take the +1 weapon after the Magic Weapon spell, and add up to +4 worth of bonus to it from the Bond. So you won't supercede the bonus until level 17, where I imagine you'll have much better than just a level 1 Magic Weapon spell.
EDIT: I forgot to address the DR question. In which case I reference:
Any weapon with at least a +1 magical enhancement bonus on attack and damage rolls overcomes the damage reduction of these monsters.
Also just to have the full ability written out:
At 5th level, this spirit grants the weapon a +1 enhancement bonus. For every three levels beyond 5th, the weapon gains another +1 enhancement bonus, to a maximum of +6 at 20th level. These bonuses can be added to the weapon, stacking with existing weapon bonuses to a maximum of +5, or they can be used to add any of the following weapon properties: axiomatic, brilliant energy, defending, disruption, flaming, flaming burst, holy, keen, merciful, and speed. Adding these properties consumes an amount of bonus equal to the property's cost (see Table: Melee Weapon Special Abilities). These bonuses are added to any properties the weapon already has, but duplicate abilities do not stack. If the weapon is not magical, at least a +1 enhancement bonus must be added before any other properties can be added.
Therefore, whatever the final +X bonus comes out to be is the DR bypass.