any idea why the GUI installer in 3.5 Openbox becomes unresponsive?
The big question is: have you used 3.5 or 3.5.1? 3.5 had an issue.
We spoke about upgrading your machine before
There is no change with that.
Upgrading after so looong time is risky, but possible.
Backup your personal files than do it in a terminal emulator:
apt-get install -f
Kernel 3.16 is in Debian testing repo so you can get it now.
Anyway you can try to upgrade the kernel first, for example via Synaptic: Reload, then install kernel 3.16, the reboot.
If something will be wrong, you can reboot and run the system using an older kernel.
If it'll be fine then try to upgrade all packages.