I would tackle the smoke quit first, and eat as well as possible during that time. When you quit smoking, your body tends to naturally purge a lot of toxins, and you need to eat right and hydrate in order to help that process. Fasting at the same time might be too much at once.

I quit smoking six years ago after multiple attempts. I highly recommend:

www.quitsmokingonline.com (a little hokey, but it works)

with follow up:
www.cognitivequitting.com
www.whyquit.com
www.quitnet.org (a community like TE.)