If your purpose is Restoring wallet.dat, you do not need to run the Bitcoin core client to obtain your coins. You can extract the private key and import it in Electrum or any SPV wallet. These wallets do not require you to download more than 500 GB.
To do this, make several backup copies of wallet.dat.
Download bitcoincore Verify signature Open wallet.dat Enter password.
Create a file and save the path.
Open console in bitcoincore and type dumpwallet <file path>
4. I want to make sure that an external hard disk is used as a data directory (where all the synced up blockchain data is stored). Should I specify this external hard disk name before or after installation of the client?
Change Bitcoin Core's default directory to the external hard drive data directory and you need to do this during installation, otherwise the data will be loaded into the default directory.