See this to me makes me think you mean money to spend on re-training in the hope of furthering or starting a career after this training.
Basically what I think you should have said is; "If you had £500 to pi** away just for fun on something learning based after finishing a job, what would it be?"
I'd love to do the ARDS course but as Alex says unless you're not going to compete, attend sessions or hire/buy a race car plus all the equipment then it is a bit of a waste of money although in saying that I'm sure doing the course (track time & classroom) would be very interesting.
Probably better off though spending the money on track days or maybe some type of sports car hire etc.
I think for the £500 I would do one of the hydro dipping courses that AquaGraphix run as I find that very interesting and in some ways it would tie in nicely with skills I have and my current career.