Just swap out the rear wheel. Once you get into the bottom bracket and headset area, issues multiply, but aside from that, your bike will not be that hard to work on.

Switching out the spindle for the bottom bracket, for example, may also require switching out the bottom bracket cups, as the cups on many older European bikes are thinner walled than the later, more common cups from Asia. At that point, you have to begin looking for parts, and where five-ten years ago it was simple, it's become progressively harder to find french thread stuff.

If you're set on switching out the cranks I'd wait a couple months. Velo Orange is manufacturing a French thread cartridge bottom bracket; it is not available yet but should be soon.