I think it would make your search much easier to work with a local bike shop. You want to get bikes that will fit you, and height is only one of the factors involved in determining the right size. I am sure that the TE members will be able to advise on good stores to visit in your area. I know this is a gift for your girlfriend, but it would be good to find some way of getting her input as well. Does she know that you are planning this?
It sounds like you are going to have a lot of fun with this, and you should