Welcome to the forum Rab. Will be very interested to see your progress on this as its something I was planning on doing too once I've got the extension finished (will it ever end!!)
Was going to see how cheaply I could build one for doing everything from scratch. Unfortunately its got no further that buying the 'Build you own sportscar' book, which is currently just gathering dust. Using freecycle could be an inspired idea.
Budget currently
-£13.50 
John