Web Based Reservation System

Hi I am looking for a simple web based reservation system where my customers will view my website, look on the system to see if a rental product is available and pay the hire fee via Paypal. Essentially the customer will look at product 'a' (the website and products are already in place) and click on 'Check Availability' this is where the system should check to see if we have any remaining units for rental. If a rental product is available, the customer will enter their name, address and telephone number and be sent to Paypal for payment. Once the transaction verification is received, the system should reduce the amount of the rental product available and enter the booking under a reference code in the system. Should another customer want to book the same product and we have no stock left, the system should show the product as fully booked. The interface should be in the form of a calendar system with an admin interface to add / remove rental stock etc.