0- lvl Magic Missile

[Elder-Basilisk]

As for the 1 pt. magic missile being useful, I can only see that at 1st through 3rd levels. After that, you're not going to make any spellcaster fail a concentration check with one point of damage. (Even at first level, you're not very likely to make the spellcaster fail the concentration check. . . .) Any fulltime spellcaster with less than maximized concentration obviously has an intelligence penalty. . . .

DR 1/+1?
Duration: at least 1 minute. Otherwise it's completely useless. Other effects would have to be a function of level.
I'd say:
1 minute/level: 1st level.
1 Hour per level: 2nd level.
It's not a very good spell at either of those levels though. It would really only be useful against hordes of dretches or skeletons (who aren't using weapons). And how often do those come up?
Weaselly players might very well use it as a weaker version of Magic Fang as well (giving a creature DR so that its natural attacks would overcome DR--assuming that feature wasn't explicitly excluded from the spell as it is with Stoneskin).

It would be tempting to make it a zero level spell but I don't think that would be wise. . . . It's probably competitive with Regenerate Minor Wounds (MotW). As a zero level spell, the closest comparison is probably Virtue. DR 1/+1 is so clearly superior to one temporary hit point that I'd have to place the effect higher than first level.

The 1 pt Magic Missile can be usefull for a few different reasons. It does less damage than a Ray of Frost, but no attack roll needed, and no saving throw. Also, it is not cold damage.
Yes it could force Con checks, and hopefully disrupt spellcasters.

As for the Damage Reduction 1/+1. What level would you think? What about duration?