2.25ish, the calculator here has values for lots of different brews, including cider.
Your priming sugar is going to ferment out to produce your co2. So, the flavor of it isn't really critical. Other then it not be something crazy like molasses or something.
To add apple flavor, it's fairly common to use apple juice concentrate to back-sweeten. The problem with that is all the extra fermentable sugar you are adding. That can lead to over carbonation and/or bottle bombs. The safer choice to back-sweeten is a non-fermentable like, lactose, sucralose, or like you mentioned stevia.
In most people's heads apples = sweet. I think you will find that if you sweeten your finished cider you are unlikely to need more apple flavor. I would suggest sucralose, specifically baking splenda, as it has a neutral flavor profile. Both lactose and stevia bring there own flavor compounds to the party as well as sweeten.