I don't think you can reliably tell where a user is located by ip address. Most web sites have you choose your country. Either way no WebAssist software will help you with this part so my suggestion would be a simple form asking the user which country they are in and storing it as a cookie and session variable for use within the site.
Then I guess you would need a database of curency symbols and exchange rates to properly calculate and display the correct related currency. You could probably store those in the session and as cookies as well.
Then it is just a matter of using the session variable or cookie to display the correct currency symbol and do the math to show the correct converted price. DataAssist from WebAssist would help you manage that database online if that was something you wanted the customer to update currency rates from an administrative interface.