Redo my cover letter and answer an interview question for me

I need professional advice on my current cover letter. Maybe redo it or just proofread and add a couple of things.

If there's a lot of work to be done on it, I am open to re-negotiating the amount to be paid.

I will give details about my profession to the person who will be doing the work.

I also need the best answer to this interview question too: Why should we give you a chance? I never have the best words for it.