Donor trailer is prolly a matter of checking craigslist. I don't know where Richardson is in relation to Dallas, but this might be a start:
http://dallas.craigslist.org/search/...k=1&maxAsk=100 In the pacific northwest, burley trailers are often listed at about $150.
Wheelbase? Good question--met someone, another parent maybe, that has a burley bee? See if you can get him/her to stop by with their trailer for an opportunity to balance your table case atop their trailer. It will be pretty obvious where the center of gravity sits, just make sure you leave a few inches in the front of the table so that the left and right corners of your table case won't strike the rear wheel when turning. Worst case, you'd line up the front margin of the kid trailer with the front end of your table case.
InStep, Burley, ...actually looking on
amazon I see trailers listed for just over $100 -- but I can't vouch for them. Dimentions might take a bit of puzzling:
Folded dimensions: 35 by 30 by 12 inches (W x H x D)
from amazon. I'd assume that most trailers are about those dimensions. Comare the "assembled wieghts" on the trailers listed on that page...it ranges from 20 to almost 40lbs. That's a pretty broad range. My guess is that I'd stick with the Burley set, it might be narrower, but with a smaller pvc rimset and aluminum tubing, it might be more appropriate for a longer commute goal. The schwin, instep, or aosop brands are over much heavier. The trade off is they probably have 20" steel-rimed wheels, which might tow a bit more smoothly, but you'd feel the difference going up hills.