How to write invitations in Japanese?

Question:

I have very little experience with Japanese and let's just say that the text books don't prepare you for the stress of summing up courage to invite a Japanese ambassador to a fund raiser for Japan earthquake victims. ;D

The invitation is very short. I suppose I start with 拝啓 and finish with 敬具, right? And the rest is all keigo?
This is the basic structure of the invitation.

Your excellence,
Due to recent events, the organization X and the association Y decided to organize a fund raising picture exhibition of the famous Japanese photographer ZZ. We would be honored if you could support our efforts with your presence.

Answer:

You are exactly right about Haikei and Keig, and as you say, you need to keep everything in keigo.

Since you are non-Japanese, minor errors won't be a problem, if anything, that might work in favor of you.

I'm not an English speaker either, so maybe it's time for me to pull out of here.

Thank you.