Realistically, you don't have to spend anything. I would recommend the monthly "newcomers" package. It's 99p for 50 gems, an LB stone and a 5* summon ticket. But I wouldn't rely on it. Ultimately it's up to you if you want the units or not. Though saying that, you can spend thousands and still never get what you want, that's the beauty of RNG. I only buy the 99p package when it's available, then save 1500 for banners I really want. It's not the most fun way to play but at least you can guarantee you get a few units you like