Changing WorldPay to Paypal - modified OsCommerce shop

We have a OsCommerce site that has been using WorldPay as its payment gateway and would like to change to Paypal instead. Our site has been running for almost a year now on WorldPay, but due to changes in WorldPay's policies, they are unable to provide their services to us anymore. The only other viable alternative would be Paypal. Our OsCommerce site has been modified from the base install to include the following functions; 1)Hire purchase 2)Rental at different rates(monthly and weekly rental rates) 3)An option to buy after renting for minimum period of time 4)Recurring payments using WorldPay's futurepay function 5)Shipping options to include self-pickup (free) or delivered (charged at a fixed rate) Other minor contributions have also been installed, which should not affect the changing of WorldPay to Paypal. We are unsure about the size of this project and have requested for the lowest bid. Please bid as you see fit.