You would have more trouble modifying PowerStore to accomplish this than using the supper suite to create this from scratch.
to be honest up front, this will be a rather large undertaking, and will require hand coding PHP and creating the MySQL database structure. as well as creating complex SQL statements. If you are comfortable creating SQL Sub queries and JOIN Queries as well as coding PHP and creating Databases it should not be terribly difficult.
You would need to create a table of the available rental cars.
and another table to hold information about the cars that have already been rented: Which car, what date date it was rented on, when it is returned etc.
When the customer selects the start and end date, you will need to check the rented table to eliminate cars that are not available.
The results page will show only the cars that have not been rented for the date range selected and the customer can place that car in the basket, then checkout.
on checkout, the information of that rental will be stored in the rental table.
