I will be posting some plans. The best wire cages are powder-coated or have the metal otherwise coated. This protects the metal against the corrosive urine and makes the cage easier to clean, and also protects the rats' feet from the abrasive metal. Watch out for cages with sliding trays, because urine can get under the tray and make a big mess. Heavy plastic pans are usually better than metal trays since they can have rounded corners and are more resistant to urine.
Upper stories are nice in a big cage, but they can reduce the options for putting toys in the cage.
Look for cages with removable shelves. That way you can arrange the cage in different ways. It's nice to change the arrangement of the cage occasionally to give your rats some variety. A company called Martins Cages has pretty good rat cages for sale on the internet. Their smaller cages can be used for travel cages, but are not large enough in my opinion for regular homes.