Sounds like fun. I've never made one but hope you do a better job than I did on my toothbar project of taking pics to share with us here on Net Tractor Talk.
I've just built a coop, no tractor. I'd suggest you look at as many as you can find, then build one in your head first using the good points from them all. And of course the first question is how many chickens. I've seen some tractors that needed a tractor to move them they were so large.
Here's a good chicken site http://www.backyardchickens.com/forum/index.php I did a quick search on "tractors" and came up with some nice pics.