It's a good idea, but sadly - the wall will go to waste. Graffiti artists, or people who do graffiti in general do it for the rush, that being criminal activity.

It's best to ask the government to set up a wall, and pay graffiti artists to paint the wall/skate part/city benches - that way the graffiti artists get some dough, recognition and provides a distraction from the usual antics that graffiti artists get up to.

First you have to establish who you will be contacting to ask for permission.
Now let's think of this proposal as a job interview. Writing it formally and with correct grammar is great, but don't overlook the other key factors. Unlike an actual job interview, you don't have the luxury of knowing what the interview questions will be. So brainstorm and think of what type of questions would be asked.

Who are the artists?
How will this be beneficial to our town?
Will this be profitable?
Who will monitor the wall?

Those are a just a few possible questions you might be asked. If you write a proposal that answers all his questions. He will be impressed; I'm certain.
Like all job interviews, you should prepare a portfolio. This should contain all of your styles of art; not just murals.

I highly recommend you do not use the word graffiti. When people hear the word graffiti they automatically think: vandalism.

Good luck- No GREAT Luck!

~SKSF~