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I agree with the above posters. It'd be a lot of expensive work to get the Schwinn upgraded to the way you want. Get a rigid mountain bike from the early 90's or even better the late 80's. An 80's MTB with its long chain stays is basically modern touring bike geometry. You should be able to find something suitable for 100-200 dollars on Craigslist. It'll already have wheels that should stand up to the loads you are going to put on them and already have the gearing you need, saving you all the expense of the cranks, derailleurs, wheels and brakes. Have a mechanic look it over and put new cables and housings on it as well as new brake pads, then all you need is racks, tires, and a saddle.

This also has the benefit of keeping her heirloom original instead of turning it into some Frankenbike.