This is a great place to learn all things Airstream. If you are looking for an older model, I am sure you can find one that will fit your needs at a price you can live with. Be sure and check out the classifieds here as well as look at what is available via ebay and craigslist for your area.
If you decide you want to expand beyond Argosy and look at a traditional Airstream, this link may help you think about year/layout/etc. that will meet the needs of your family. (scroll down to see years/models)
Vintage Airstream Photo Archives
Here is also a good place to look for basic price information. It will give you lots of information to think about! Take this information along with current trends and you may get a fair idea of what you may pay.
Price vs. Condition - Airstream Values
You will also want to determine how long/much of a trailer you can tow with the vehicle you own. There are lots of threads on vehicle type/size and trailer type/size, check out the search button on the top right of the blue toolbar.
Great to have you aboard! Good luck in your search! Folks here are really great!
Mrs. NorCal Bambi (traveling in S Tardis ~ from the Great State of Jefferson)