About LC: buy only Gold and Rainbow skill bloom, the few Dress Bloom (becareful, they are limited and never refilled) and the 5*6* ticket (never refilled too). All the other stuff don't worth theses LC at all. Silver skillbloom could be useful to reach skill lvl5 on a 4* you REALLY want for Rainbow promotion.

About Bond Crystal and D-medals: Don't buy the girls with D-medals. D-medals are too rare. Use them for tickets or Rainbow shards if you really need them. Use Bond Crystal to buy theses girls, if you want them. You can get a lot of Bond Crystal through many ways (equipement reforging, selling/using as upgrade material 3* to 6* girls, daily quests, on certains events and through some campaigns).