bitaddress.org already provides your private key in Wallet Import Format, which is encoded in base58check format.
In the example screenshot below, the base58check-encoded WIF private key is
To address your “rule 1”, the most secure way to handle this data is load Electrum onto an airgapped machine, enter the private key into that secure computer, and then use it to sign a transaction you build with an “online” machine that has access to the UTXO set and your wallet address.
Warning: Do not use the key or address in this example for any reason at all besides looking at it and learning! Pasting private keys on websites is always a terrible idea.