How would I architecture Direct Attached Storage?
DAS means you connect one or multiple external hard discs locally (e.g. by USB or SATA)

1) Then, you share them from this computer. What you need are standard external USB drives and a working home network. 

2) You can share them using any Mac that is "always on" when the others are on

3) no, standard Mac OS will do

4) OSX. System preferences->Share->File Sharing

To connect, in the Finder select "Connect to Server..." under the Go menu.