One Page Checkout and Built in Email Sign Up Form

I would like a One page checkout on www.NaturalPetWarehouse.com. It would need to have the look and feel of the rest of the site (header, footer, colors, etc). I know [login to view URL] has a one page checkout program, but if you know a better add on then I am open to suggestions. I do not want customers to need a login to purchase. Right now if a customer wants to order without a account and has already made an account, they cannot because their email address is on the database. Also would like a email sign up form intergrated into the top of the page somewhere that it would look decent. I only want the customer to place email address, no other info. My site is also running in quirks mode. Wondering how much work it would take to fix whatever bad code there is to get me in standards mode with good css. Please include how you would need to be paid. Thank you, Brandon