modify existing website or create a new module for site.

I currently have a website that I need to modify. My business is a limousine hire service. The part of my site I am modifying is am online quoting and booking section where clients or new customers will select there pickup and destination that they are wanting to travel to and select the vehicle type also select the date and time as this may change the quote.. this then gives them a quote based on the details entered using google map data and a spreadsheet that I can modify information like the cost per kilometre and other variables. the next step after the client gets a quote is if they want to go ahead and book the trip. After they enter all the required information they give there credit card details and get a confirmation. My job I would like to post Is to go in and modify or create a new system to do this process. Modifying would be to make it more user friendly and fix any bugs in the system. or to start from scratch and change the way the system works. website is currently at [login to view URL]