C&C cages are really not the best for rats, though with modifications they'll work.
If you do line grids with wire mesh, be sure that you use a wire that is not galvanized or zinc-coated. Rats may chew on the wires, or climb it and then lick their paws, and they can get heavy metal poisoning. The wire must be stainless steel, pvc-coated or powder-coated.
Rats, like guinea pigs, should not walk on wire, so be sure every level is covered with something hard like ceramic tile or coroplast. Fleece or towels work as a bedding, in conjunction with litter boxes using the same kind of bedding as for pigs.
I've seen people use the mesh panels for a cage for rats. I haven't ever seen these panels in person -- I would worry about rats getting their claws caught in them. Mesh that fine is usually not recommended.
I agree that the Ferret Nation is a great cage!
Check out this rat palace made with one! However, only *fully grown* rats are okay in it without modification, as it has wide bar spacing. Some females are never big enough. However, it can be be modified as well, with safe wire mesh.
Just this month, however, the same company released an alternative! The Critter Nation is the same cage, but with smaller wire spacing. It's brand new, so I don't have an experience with it. The main problem people have had is with the cage being damaged in shipping, though the company has been very good about sending replacement parts. They should sell it at PetSmart soon.
The other option is building a cage out of a cabinet.
The Dapper Rat shows their own cabinet cage, the Grotto, and how to build your own. It's a great rat site with lots of information.
Another great rat community is
Rats No SPOs.
Rats also are very social animals, like guinea pigs, so once she has an appropriate sized cage, she probably wants to get her rat a companion!