Limited time content pays the bills. If this tank is important to you, you do what you can to get her while she's available. (And worry about the candies for her later.) Some people who are on the fence about spending gold on this game will decide to do so in order to collect her. Some, of course, will not.

If you're totally F2P like I am you should simply accept that you're not going to get most of them. F2P and completionist are largely incompatible concepts.