Levels and bars will need to be cleaned effectively at each cleaning or they will be corroded. Levels can be replaced with the plastic bowls I described earlier. Make sure you construct the combination so that you can get the mice out of the cage easily. You can use clips to attach the cage part to the tank, so that it is very easy for you to separate the two for cleaning purposes. Tank part is heavy and breaks easily - unless you use a storage box.
Mice can enjoy climbing the sides of the cage.
Cage top gives better ventilation than just the tank part and the tank still provides warmth and protects from draughts. Cage parts can be very cheap or even free. Remember - you don't need the tray at all. The tanks parts are easy to find. Better interaction with your mice than with tank alone.