Hello!
I've read over your requirements, and everything is clear. I have also looked at the theme on ThemeForest, and WPEstate supplies a ready-made child theme included in your package, which is my likely path to making the booking revisions (Using a child theme will allow future theme updates without breaking the changes involved in this project).
While I haven't worked directly with WP Rental, I've worked with many other themes and plugins, including building from scratch, and am confident that I can complete this as well.
Thanks and hope to be working with you,
John