S.,
How fun that you are going to share your love of biking with your daughter! This is my advice, go try them in the store. Place your daughter in them and see how she fits in the restraining system. We LOVE our bike trailer but have problems with the restraining system and had to add pillows in the back and under them so that they wouldn't slump down into the straps. Take it for a spin around a couple of the Ailes and see how she if fitting. I wish that I would have done this before I bought mine.
Another piece of advice, if you buy used check with the bike strollers manufacutre to see if there have been any recalls on it. I do this all the time with my used purchases off ebay and craigs list. They are happy to tell you if it has had a recall & if it is fixable if it has had one. Buying used it a great way to save money! Good luck with your search and enjoy your rides!!!