I have a SAGE with the scheduler feature added, and a docking station. However, they (the models) all seem to have about the same vacuuming power...the different models just come with different features; and you can always add many of those features later.
The unit works well on carpet; what you have to keep in mind is that while it may not have the suction of a vacuum cleaner, the unit will travel over and over the same floor surface many times in the hour or so that it runs, thus giving the carpet a cleaning that a brief vacuuming might not attain.
The roomba does require cleaning, though...more frequently than changing a bag on a vacuum cleaner. It can be done on a kitchen counter and is not very complex; after doing it a few times, it is easy. And the roomba is pretty light to pick up.
The roomba will likely need to be moved from room to room by hand, but you would do that with a vacuum cleaner, too.
The scheduler feature can start the roomba at a pre-programmed time, or you can start it manually with the remote and steer it to the room you wish to clean...programmed cleaning works well with an open floorplan where you can place it centrally in a room with open access to another room. The roomba is not very noisy, but you woulldn't want to watch TV with it operating in the same room.
If she is open to the idea, she'll probably like it.