Ecommerce Site for downloadable products

I would like a website creating to sell web templates similar to templatemonster.com, This site will be selling ready made website templates in 4 main categories html, Flash, Ecommerce and Freebies.

As with template monster I would be selling templates on a price per purchase but would also like to offer membership giving customer access to all templates for an annual fee.

In addition to this I would like to have discount code integrated with the checkout process which will give users 1 months free access when they enter a discount code.

On a cost per lead basis once the customer has finished the checkout process they will be presented with a download link to download a zip file.

On membership basis members will be able to click download on any of the templates.

I have already created a basic html version just to show designers and let them see what I am looking for.

http://templatemarket.co.uk/Ecommerce%20Templates/001.html

I would like it writen in PHP / Mysql and integrated with Google Checkout (however if this causes any issues paypal will do)

I am egar to have this completed ASAP as I have already been working with a designer who has taken a deposit some weeks ago and now can't answer his telephone.