mei there costs way more than 1535 coupons (something around 7000/8000 coupons). If you are a f2p i would suggest you to not go for her that way.

My suggestion is to put around 120/140 scrolls in kage treasure and look at what you get and change main based on which two rares you get there, but could be you can be compelled in going on with another 70 or 140 scrolls if you don't get a synergied duo. Otherwise you can aim to the jinchuriki lineups and go all the way in jinchuriki 1/2 treasure to get both han and roshi + all the common rares there, will take about 440 scrolls but you'll have for sure a meta team, a ranked team and some good fun lineups with azure fang to try.