|To start you never want to put a test domain in your existing forest/domain. That is asking for all sorts of trouble as you conduct tests and change configs.|
You would start with an internet gateway router that could support multiple subnets. SOHO routers can't do this.
You can use all the same hardware ie. switches etc though it is best if you vlan as well as subnet to provide a true barrier between the two networks.
You would bring up a new Forest and domain in that forest. This would require at least one DC.
And there you have your test domain that you can trash at will [then recover] with no impact on the live company network.