Just thought I'd update now that we've been using this tractor for a few months. For the most part it's great. If I did it again I would not put the run access at the end of the run hinged at the bottom. It's hard for DD to get into the coop without stepping on the door. Instead I'd put a hinged door on the side of the run, maybe next to the house so you could close the pop door without needing the drawbridge-style ramp we're using now (which does work fine too). Ditto the egg door; side hinges would've been better, I think. The lawn mower wheels were fine when the tractor was in the front yard, but in our bumpy, root-ridden side yard I think bigger wheels might be better. The hardware cloth and 2"x4" bottom wire have kept out predators, we've had no losses to the local raccoons. The bottom wire does tend to collect the hay bedding the girls pull out of the house, so I have to rake it out once in a while and put the goodies in the compost pile. We have 8 bantams in it, but I think 6 would be better. They're not picking on each other at all, so I guess they're not technically too crowded, but once we build our second doghouse-tractor I'm planning to take out 2 girls and move them in with the 4 new ones.
Hope that helps anyone hoping to build a similar tractor! We do love it, and it does its job well. It's definitely a major attraction in our urban neighborhood; our girls have several regular visitors.