Hello and welcome to the forums!
The answer to your question will certainly be one you will need to decide upon. So many things to consider. There are some threads that cover this topic and you might want to surf and see what you come up with! There is one about things to consider when buying an airstream. I don't have the link right now, but I will try to find it and post it so you can read it. Many of the topics are about newer airstreams, but the issues are often similar.
Probably the two biggest items, will be cost and what will you tow your airstream with? Do you already have a truck, etc.? The load capacity of your towing vehicle will help you determine how much you can pull.
If you are looking for a 70's type year, then you will need to decide if you want to spend the money for one already in good shape, or fix one up....
The layout and design will be totally up to what your family likes. Many folks have created their own interiors or added things to their liking. I often suggest that you search everywhere and do your homework first! There are some excellent vintage airstream sites that list values, ebay certainly gives you an idea of what folks are paying. Often the pictures on ebay are a great place to print out things you like! Craigslist.com also has airstreams if you key it in the search box for different areas. Our forum site also has classifieds! Look at them all!
I would also consider what kind of camping does your family like? Do you like forest service campgrounds or RV parks with all the amenities? The size of your trailer will be a factor for some roads, parks, and campgrounds. Many places will not fit a trailer over 25 feet in length if you like the out of the way places.
If you haven't looked at many airstreams I suggest you check out the calendar of events and rallys link at the top of the page. Find a rally/event near you and attend. Folks are often really happy to show off their airstream.
It will give you a great opportunity to talk to people and get their opinions and find out the pros and cons of their size/model. You can also go blurry eyed and read the forums and get lots of information!
I would make a list of things to check out, discuss them with your spouse/family and see what things will work for you!
I must say that the very best thing about airstreams are the great folks you will meet! The forum rallys are wonderful, fun, entertaining, and easy going!
Good luck on your search! Again, welcome to the forums and post all questions you have!
Mrs. NorCal Bambi (traveling in S Tardis)