If you use Rare Candies on a Pokemon, the Pokemon won't gain any EVs. So if you Rara Candy right up to LV.100, then It won't have any EVs, making it a lot weaker than a Pokemon trained in battles up to LV.100. So yes, you will have to breed another Espeon and start from scratch. The Daycare is just as bad as Rare Candies, they don't give any EVs either, so the quickest and most efficiant way to get a LV.100 is to train it yourself.