I'm not sure what you mean by older, but other than maybe Jack taylor, most pre-80s tandems, and nearly all production tandems, are too flexy and too short at the stoker position with too short a wheel base. With the height issue you describe, that problem will be more pronounced. I'd stick to the tandem big guns - Sterling/Bilenky, santana, co-motion, burley and Cannondale. Stay away from the older Schwinns with her height. I'm sure I'm leaving some out, but you get the idea. Recently rode a peugeot tandem for a couple of miles down the shore - what an incredible piece of crap, but it made me appreciate mine all the more! I've seen treks that look good.

Bilenky makes a view point tandem - captain in rear - but they are $$$$$ and you'll still likely have the same fit issues buying one used. I think buying one used is going to require some fit compromises and playing with stems, posts and bars. You'll want a longgggg rear, a high seat post for her spot and a stoker stem that has a LOT of angle up. My stoker stem (going to Tom) was very adjustable on length, but you're going to want some angle to it.

I think you can make it work with a relatively standard sized tandem and some tinkering. Along with grounded expectations! My bet is you'll go custom if you find you like it. I was talking to Simon @ Hanford cycles about building a tandem, and he was talking pretty reasonable pricing compared to the others - and he's very good. He built his own view point for him and his wife and it's a beauty of a bike.