I would look for a used Santana, CoMo or similar tandem. I recently sold our Santana Sovereign for $2000. That's a pretty a good way to get into tandeming considering it was in excellent condition and new one costs over $5K.
Tandem magazine classifieds is a good place to start. I feel a rear disk brake is not needed unless you are a heavy team, will be doing loaded touring or riding a lot of long steep grades or some combination of those.