Just steal Woody's http://www.diyelectriccar.com/garage/cars/243What we need help on is battery choice, size, cost and amount as we are struggling to find solid information dealing with this.
A member called Jim Dear (no relation to John Deere) also is into tractors. And I have a small (& rusty) one. http://www.diyelectriccar.com/forums/showthread.php?t=44430&highlight=etractor
I don't think anyone here has done exactly what you have in mind, but use the info section http://www.diyelectriccar.com/forums/showthread.php?t=669 and search feature and you can find a lot of basics which will apply.