Hmm, I tend to avoid having NPCs who lead the PCs through a pre-plotted adventure. I may sometimes have an NPC adventurer with the party who can provide snippets of world info, give me a chance to roleplay in a combat-heavy game, maybe provide a romantic interest for one of the PCs. Depending on class and the stage of the game, it may be ok this NPC to be higher level than the PCs, as long as s/he doesn't overshadow them. Generally, I've found that it's fine for the NPC to be stronger defensively - better AC, more hp and better saves than a PC are all ok, as long as the NPC is not putting out more damage than PCs of similar class. So eg an NPC Fighter should typically have STR lower than the party's 'tank' Fighter.
As far as quest-giver NPCs go, I have two approaches.
1. In an open/sand-box game, these need to be actively sought out by the PCs, after perhaps an intro scenario where the NPC brings the PCs together for a mission. Sand-box play absolutely requires pro-active PCs; successfully finding a quest giver is a reward for good play. These quest-givers can vary a lot in power, reliability and motivations.
2. In mission based play, the NPC quest giver is the boss of the PCs, mostly offstage. They brief at the beginning of the mission, reward at the end. An example is King Thongar in my Willow Vale game, the PCs are his champions. In this case he is "once mighty, now aged" explaining why he doesn't do everything himself. He's generous, reliable, concerned for the lives of his men. He sets a task - sometimes giving the PCs a chance to turn it down, if particularly dangerous - and leaves it up to them how to accomplish it, though he may give aid, eg a squad of Royal Guardsmen. I try hard not to make him obnoxious or overbearing. One thing I use is the "power of weakness" - emphasis that while Thongar used to solve the Vale's problems, the PCs are the Big Damn Heroes now, without them there is no one else, and without them the bad guys will win.