I'll give you a response based on experience, not based on the minimum requirements. If you've got a network of 50 users, and you want to be able to run many of the great features (intrusion protection, a/v for web and email, etc) then please consider buying a cheap P4 machine, 2 Ghz or better, with 512MB of RAM. As far as hard drives go, although Astaro can run on a smallish drive, there's really no value in buying a smaller drive because of the way drives are priced in the market, and you certainly don't want something old and used. A standard 40 or 80 GB drive will do nicely.
