we have 26" wheels and 1.25" tires on our tandem. I LOVE the stability.

I think steel is a good way to go. We have friends who have spent THOUSANDS on ultralight tandems and they have so many problems.
you're hurtling through the air at speeds up to 60mph and you weigh combined about 3-400 pounds (loaded) you don't want to scrimp on structure here.

Good luck finding the bike used. I don't think you can. Unless you get something like what we have.