Just to be sure asking here:
i bought a second hand cubase6.5, its in the elicenser eligable for c7.
Wondering now how to install:
Do i just install with the 6.5 dvd, then on top of that: core installer Cubase7.0 and then Cubase7.07 installer
and on top of that. And after that the 7.5 update (that i still have to buy)? 
thx in advance
You have to download the cubase7 DVD from steinberg and then install the updates
You can keep all versions on your computer, the executables don’t take much space. Or, if you plan on using only 7.5, you can download it here and you don’t need to install 6.5 and 7:
http://www.steinberg.net/en/support/downloads/downloads_cubase_75.html
The only important thing is that you have a valid 7.5 license on your key.