This site has great Da Vinci information.
LEONARDO3 Home ENG - Leonardo da Vinci | OFFICIAL WEBSITE
Including many of his machines
LEONARDO3 - Leonardo da Vinci | MACHINES COLLECTON - COLLEZIONE DELLE MACCHINE
And a book about his robots that includes models (the self propelled cart is included at 1:4 scale).
LEONARDO3 - Leonardo da Vinci | STORE - NEGOZIO
I would love to see this as a paper model, one of the books has a paper model in it but it is basically a box with flat wheels attached. The accurate wood and steel model is more what I had in mind.