I am going to install the 3.0.5 Installer and move the old wallet here. This will be on the off-line. I assume I have to upgrade to the same version the on-line pc as well. If yes, will I have to use the wizard and the public key to create my watch-only ?
Yes, it really pays to make sure that the off-line and on-line wallets are on the same version... sometimes things get "upgraded"... and older versions don't understand things that newer versions are doing and issues arise. In 99% of cases, it probably won't make too much difference if the off-line and on-line versions aren't the same, but better safe than sorry 
To update the online version... if you already have the watch-only wallet on the old version, you can just do the same procedure as the off-line... install the 3.0.5 installer and then move the old "watch-only" wallet into the wallet directory for the new version.
Otherwise, just create a new wallet and use the "Standard wallet -> Use Public or Private Keys" options and then copy/paste your "Master Private Key" (aka 'xpub') from your off-line wallet. That will generate your watch-only wallet.
Ok thank you
