Are you saying that you are just going to use the base pan from the old cage and build the entire cage body out of the galvanized wire?
You will find that a lot of people will say you can put a whole bunch of mice in a cage that size. I wouldn't be surprised is someone recommended up to 10 mice. Personally, I, like you, prefer to err on the large side for cages. I think that 3-4 females would be perfect. I have a cage with roughly the same floorspace but that is 36" tall, and am currently housing a single buck (mouse) in there, and it doesn't seem excessive at all (too me, at least lol!).
I doubt you will have any problems with the bar spacing- the only chance would be if you have very small, young does. My cage has slightly smaller spacing than 1/2" x 1/2", but not by much, and I have never had an escape.
I am unsure about how much you are really going to want that flip-top lid. It can be hard to catch mice from a side door when they are still new and skittish, but they should warm up to you eventually (I recommend making a habit of giving them treats through the bars). When I used a tank for my first three girls (worst idea ever, btw), I obviously had to reach in from the top and they were always terrified when I went to catch them. When I switched them to the cage, the change in their behavior was quickly apparent- they were much more willing to interact with a hand coming at them sideways than the previously looming-overhead-hand-of-doom.
Also, the plain galvanized wire might not hold up for long. It can get nasty and stinky pretty fast when there is pee all over it. I'm pretty sure that there is some type of spray you can buy at a hardware store to do an at-home powder coating, which will protect the wire from all the nastiness.
