What is the minimum required set of parts for Mindstorms? I'm interested in trying a second-hand Mindstorms kit, but can't afford the full kit. It doesn't need to be the latest version, either. If compatible with my computer, RCX would do.

Is it possible to learn the basics from a subset of parts from the kit? If so, is there a recommended set of parts to do so?
I would get the NXT brick itself, at least one but preferably two motors, and the touch sensor.

Obviously, you need a brain brick, but you also need a way for it to do something, thus the motors. And if you want to do anything particularly interesting, that doesn't need to be automatic functions only, you'll need a sensor to start different functions at the proper times and in the proper conditions.

You could expand a little by adding the motion detector, but mostly you just need an input, output, and brain.