If it's going to be a priest, spec shadow to get at least Mind Flay, Shadowform, Vampiric Embrace and Vampiric Touch. Then get those imp buffs from the disc tree and go for Divine Spirit/Imp Divine Spirit (I'm not sure if the talent points are really enough for that). You get the buffs, you get decent damage. However, that is a weird build. But it will give you what you want.
And "having buffs" for yourself isn't the point here. You have them anyways, you can improve them, but the improvements (except imp Inner Fire) aren't really that important if you don't want to group up with anyone.
By the way, even if you are a Shadow priest, you will still get asked to main-heal instances quite often until 65ish.