The QuarkXPress 6.5 is not available for purchasing, but you can upgrade to the latest one which is QuarkXPress 2017 for 399 Euro. This is the price for versions 3 - 2005. You have to visit the official website and mark that option then follow the instructions to upgrade to the latest version.
You also need to meet the following requirements in terms of Mac OSX versions:
- Mac OS X 10.10.5 (Yosemite)
- Mac OS X 10.11.6 (El Capitan)
- MacOS 10.12.x (Sierra)