Can magic circle against evil be dispelled by a simple dispel magic? If so, would the caster have to target an area or a specific target?
Dispel Magic ends ongoing spells. Magic Circle Against Evil is an ongoing spell with no particular protections against dispelling. Therefore, it can be ended with Dispel Magic.
As for the targeting, MSAE targets a specific creature with a range of touch, and thereafter emanates from that targeted creature. Therefore that creature is what you would need to target with Dispel Magic. Targeting them directly with the single-target usage of Dispel Magic and catching them within the area of the AoE usage both work, subject to the respective limitations built into those usages of Dispel Magic.