We plan to update one of our existing car rental platforms Motion Drive from an old no-name CMS to a WordPress version - We also developed a booking calculator a few years ago for the available cars depending on date, options etc. - is there any tool or plugin that could be used for it? I only found booking tools for events and/or hotels.
Sorry but your site is looking little outdated and you will need some redesign and if you have problem with current platform take some other CMS.
It is not problem to make a plugin based on your old site if you have some budget.