Here's a thought - why not contact that tandems builder? I scroogled up his linkedin profile as top hit here and reckon he'll tell you a fair bit about the thing - personally I think I would go for it, but hey what do I know?

Really sorry to hear about the loss of your tandem - that may be another story, but should we be warned about this "con"? However one thing - I think you should consider the tandem you were used to - you will want a close match in terms of quality maybe? Or do you want an upgrade? then see how this one compares? Of course if budget is king then the only risk you are taking here is the one you already worked out for yourself....

A close up picture of the chain set would enable evaluation of the need for a new transmission.... And surely she knows how many miles on it? If you can trust her on that it is the most valuable info...