I don't have a reccomendation for the OS software you asked about however I know for a fact you can Make your Own External Harddrive, What you do is you find an External Enclosure you like, then you go out and Buy an Internal 3.5" Drive that fits your needs.
I did it, with a Maxtor 250 GB HD (Maxtor I know.) And An External Venus Enclosure. It was alot cheaper then if I wanted to buy a pre made one with the amount of space I wanted.
Use Neweggs Search feature you find a good enclosure. You don't have to buy it there..