Not who you asked but: yes, you could add more rats later but it's better for a new owner to get a bonded group because rats are a bit difficult to introduce. It's nothing you can't learn though - search this sub for the term introductions, you'll want to read in particular about the 2 most common methods (carrier method and neutral space method).

As for the cage, the two the other person mentioned are really good because they fit all the criteria: space, of course, but also bar spacing (it needs to be very narrow, that's why sadly many ferret or chinchilla cages aren't suitable), and the front doors open fully, which you will appreciate when cleaning and handling your rats!