And in case that my off-line wallet machine is not working anymore, can I still recover my complete bitcoin balance using the seedphrase which I created on the off-line machine in the beginning?
Yes, plus any additional seed extension words you may or may not have chosen when you set up your wallet. (You had to click on "Options" and then check a box marked "Extended this seed with custom words". If you did this, you also need these words in addition to your seed phrase to recover your bitcoin. If you didn't do this, then don't worry about it, and your seed phrase is all you need.)
Bear in mind that if you are recovering your seed phrase to an online computer, rather than another offline computer, then your wallet is no longer a cold wallet and is at the same risk as any other hot wallet.