For Nadia, the best purchase would be an elliptical trainer. Ellipticals provide a low-impact workout that minimizes stress on the knees and joints while still allowing for effective cardiovascular exercise. This can help her maintain her cardio endurance without exacerbating her knee issues.
In contrast, the treadmill may lead to more impact on her knees, and the other options, like a plate-loaded barbell or a nautilus machine, may not focus primarily on cardiovascular fitness.