Customise an existing script

I need the following features to be added to an existing booking engine: 1. Allow a slot based booking feature, this feature will allow me to allocate a maximum number of bookings per hour. For instance, between the hours of 8-9am only allow 2 bookings and between the hours of 9-10am only allow a further 2 bookings. The number of bookings allowed per hour slot should be customisable in the backend, the number of items per booking should also be customisable. in admin i.e for the 8-9am slot allow 2 bookings with 3 items per booking. This feature should not affect the overall availability of items for that day, (Notes: Currently the system allows any number of bookings based on the availability of the items) 2. Allow half day closed feature. i.e Close the morning (before 12 midday) to stop bookings but allow the afternoon (after 12 midday) to carry on allowing bookings. (Notes: Currently the system only allows to close a full day) 3. Allow the system to calculate extras on a one off charge basis. (Notes; Currently the system calculates extras on a daily rate based on how many days have been booked.) My references based on the scripts original purpose: Items=Cars. Availability=Number of cars available that day. I will be using this script for another purpose. The script is available here: [login to view URL] I will supply the script to be customised. Thank you.