I recommend you take a good look at osCommerce (
www.oscommerce.com) - it's free, well supported and very very customisable.
With regards currencies - it can handle an unlimited number of them and as well as customers being able to select currencies themselves, the stores can also be made to automatically detect the customers location and show prices in the correct currency from the start. Exchange rates can be updated with one click (in fact it's possible to set it up so that it automatically updates the exchange rates on a daily basis).
Hope that helps
Ali