A Cubase license can run all previous versions. You could just buy Cubase Pro 9.5 and then contact support to receive a Cubase 5 or 5.5 installer.
When not collaborating with this person, you’ll be able to open your Cubase 5 projects in Cubase Pro 9.5, which is a much, much faster DAW to work with (using a newer OS is recommended for 9.5). Note that projects saved in Cubase 9.5 can’t be opened by Cubase 5.