Costs...yes you can easily spend 5K+ for a new Tandem.. but you can also spend $1K to $2k for a very decent used Tandem that will give you many thousands of fun miles if you take care of it.
Tandems are a very very small piece of the huge bicycle pie. But there are plenty to be found with a little looking. For used try Ebay, Craigs list and any local large bike shops in your area. For more places to look and learn about Tandems in general, try
http://www.thetandemlink.com/ ..it has a wealth of info.
Can your wife Captain the Tandem?....If she has the strength to manage you team's weight and, the desire and ability to do the work required of the captain....absolutely.
The best thing your team can do now is do your homework first...like you have started doing here. Read everything you can on this site and referenced links... you will lean a lot that will save you time, money and grief. Enjoy the experience..Good luck.
Bill J.