You don't need anything fancy to only store games installation files, these are just some files that will stay on hold until you decide to use them, just get a normal drive with 7200 rpm and 64 MB cache (1TB-2TB), hard drives like Velociraptor is used to install games on them to play or work on huge video projects, if you're going to install the games on your 250GB SSD then you'll be fine, so the bottom line is:
INSTALL GAMES >> SSD
STORE GAMES ISOs >> Any Hard drive with 7200 rpm and 64 MB cache
Good luck.