most people get up to Floor 200, as that opens up enough loot for early builds of any kind. at that point, you need to decide if you want to continue climbing floors, farm for loot, or pick a Perk and start Ascending again. usually, the choice is to farm for enough loot to make Ascending for Perks later easier.
this is a post of what Crystals can be used on Eternal items. hope it helps.
Challenge Maps are gotten by defeating Cartographers or buying them in the shop. the ones from the Cartographer will be for the next Floor, and the ones from the shop will be the same Floor you are on when you buy them. also, the higher the Floor you are on, the higher the cost of the map.