Hardware to run ROS on a PC ?

edit

ROS itself does not 'need' any sensors, as it is a framework that programs and drivers can communicate on rather than a program.

If you want your robot to interact with the world, then yes your robot needs sensors. In general the more sensors, the better. If you only want to simulate the robot, you can with RVIZ and Gazebo (or turtlesim for starters).