Here's a site which lists the ages of various models: http://www.siufai.dds.nl/Rollei_History.htm
So cheap Rollei's may be old Rollei's. The Rolleiflex is a better camera than the Rolleicord, but only from an operational point of view. Stay away from the various automatic models, they didn't work well.
Yashica's are probably much younger. In addition you can keep your eye on Mamiyaflex which even has interchangeable lenses. It's a lot bulkier though.
There are even some current cameras in production; the Seagull from China is inexpensive even new. People who use larger formats tend to become passionate about this.
I see there are a number of web sites devoted to the subject, you might poke around and ask some questions. Try this to start: http://photo.net/bboard/forum?topic_id=35 Policies not Politics ---- Daily Landscape
