You can always find out the details about an old Mac by looking at http://www.apple-history.com/ or http://www.everymac.com/ or the Mactracker app.
According to Mactracker, the Macintosh Classic ran System 6.0.6 through 7.5.5. Apple won't sell them to you since they are at least 25 years old, so you'll have to find them through an online seller like eBay.
The keyboard and mouse are connected by the old ADB (Apple Desktop Bus) standard, so today's USB keyboards and mice will not work, and neither will old PC keyboards or mice. Again, you will have to find used ones online and make sure they are Macintosh ADB.