Sounds like you have a 5-speed freewheel, not cassette; which means your (her) drive-side bearing race is considerably farther inboard than a cassette hub; which is probably why you have at least one bent axle. If you’re bending axles unloaded your wheels are not fit to tour. Those old spokes also tend to lose tension over the decades. The wheels may be true now but disintegrate a few miles into your tour. Your cheapest option would be to re-tension the wheels and replace the hollow axles with bolt-on solid axles.

Drum brakes load the frame and fork differently than fork crown and seat stay bridge mounted rim brakes. I would do a lot of research before I put drum brakes on a frame and fork designed for rim brakes. Maybe you can get by with just upgrading the leavers, calipers, and pads.