The caster level of the spell is set when the potion is created
This is covered under the Brew Potion feat. When the rules state that you are both the caster and target of the spell, this is for purposes of spells that refer to the caster in their effect.
Typically, potions generated by random treasure tables will have the minimum caster level for the spell. So remove disease potions will typically be caster level 5.
You CAN create potions from offensive spells, but they're not super effective
There's no restriction preventing you from brewing a potion of Magic Missile (it's a spell of 3rd level or lower, which targets a creature or object). But remember that you are both the caster and the target of the spell once you consume the potion... In other words, anyone who drinks the potion will take 1d4 + 1 points of force damage.
Eric B notes that the magic item creation rules for potions forbid you from creating potions of spells that have a range of personal.
Hey I Can Chan notes that the Brew Potion feat does not allow you to create oils (although allowing Brew Potion to do so is not an unreasonable house rule).