Getting a "cheap" AS is usually the most expensive mistake a person makes. Since your plan is to gut one, that and length should guide your search. Can a family with two young energetic boys survive for a month in a 19-21 foot AS? Yes, if you decide before hand which parent is going to "take the rap" and "Do the time."
The longer trailers, being less desireable for many folks, are thus cheaper. My wife and I love our space and comfort. We went to a local AS dealer, and spend hours "trying out" the different lengths... thus we went with a 32'. I suggest you do likewise. Take the munchkins and spend a few hours looking at different floor plans and lengths. Try to visualize not only travelling, but living in it and not just for a few days, but for a month at a time. The differential cost of towing between a 20' and 34' will be the least of your concerns.
Then, read the forum here, it will be a great help to you.
Good luck, and happy hunting!
