Choose the exact version number you require.
F5 provides an .md5 file for every ISO (the file you also downloaded). The .md5 file contains a unique string of characters. You need to use an MD5 checksum verification program (many are available for free for Windows, macOS, and Linux) to check that this string matches the one generated from the downloaded .iso file on your computer. If they do not match exactly, your ISO is corrupt, and you must download it again.
No. F5 does not offer free perpetual ISOs. You need a paid support contract or an official trial request.