In terms of rating points, how much stronger should a coach be than their student? I'm considering getting a chess coach with the general goal of reaching the WIM title (2200 rating, plus norms; or get lucky in an Oceania Zonal).  I'm not sure how realistic this is.  However, almost everyone with a title nowadays considers themselves a chess coach, so I should be a bit selective.  I have a few questions, but my first is...

Question: In terms of rating points, how much stronger should a coach be than their student?

My current rating is 1792 and in a recent tournament in China (after two years without tournament play) my performance was 1889, including a draw with an FM and a win against a 2247-rated player.

So I have concerns that a coach around the 2300 level might not be able to help me achieve my goal.  But I could be completely wrong about this.
So, you are about 1800 and you are worried that a coach with a rating 500 points higher than you will not be able to give you the help you require?

I think you are too fixated on rating. What do you think a player rated about 2400 looks for in a coach? Do they also "need" somebody rated 500 points higher? While he was alive IM Mark Dvoretsky coached grandmasters. How was that even possible?

A coach who can help you improve by several hundred rating points is not going to do it by magically transferring his extra rating points. He will do it by teaching you how to study and work better, by directing the work that you do and to some extent by motivating you to do the work. 

When you sit down at the chess board the moves that come out will reflect the work that you have done, how much, what type, not the rating of your coach. The most important factor your coach has is how good a teacher, trainer and guide he is not how strong a player. Look for these qualities first.