Assuming you two are of average proportions, it may well be easier to find a bike that fits with her as captain. Typically, women's inseams are longer than those of men of the same height. And she's taller. But the length of the stoker compartment will matter, as one made to fit the average woman will have less length for its height than you would want.

As for expenses of adjusting a bike to fit - expect to need to replace one or both stems. The stoker stem may be adequately adjustable, but if the bike is large enough, the captain's stem will probably be too long. For a female captain, and a stock frame, you're likely best off going one size down (in the captain's compartment) with a high rise stem (or longer steerer tube, if you're replacing the fork) and the seat post cranked way up. So you can figure at least a stem. On an older bike, replacing the stem may mean retaping the bars, and replacing the cables (so they're long enough). A Santana stem is $79 at Gear to Go, higher at Precision Tandems. Plus shipping in both cases.

So unless it fits from the get-go, assume at least $100 just to get it to fit.

With the other brands, there's a much larger selection of stems, so the price may not be as high. But you may still wind up needing longer cables, which means retaping.

As for expensive replacement components: tandem rated wheels come more dear than equivalent range wheels for singles. The drum brake is getting harder to replace, as there are fewer and fewer around, but you shouldn't have to replace it either. They last a very long time. If it doesn't seem to be working, you can open it up and figure out why.

Most of us with V-brakes or cantilevers like to put on top-end brake shoes.

Tires, chains are a consumable. If it's been ridden too long without changing the chain, you're into a new cassette, and if ridden way too long, new chain rings. But those are both the same as on singles, just faster-wearing.