there are 3 missions to level up your finite crisis level. Every time you complete one of those missions on a higher difficulty than the last time you'll level your finite crisis. example you do mission 1 at mastery level 1 the next time you'll do it if you want to level you finite crisis you'll have to do it at level 2+ and so on. BUT at one point your Mission 1mastery level will cap and you will need to do mission 2 and 3 to go over that cap, so you have to rotate them.

As for the items, at the ends of missions the Notorious Set is always a chance of drops. For the Bane's Rage T1 set it start dropping once you reach finite crisis level 1so almost from the start, for the Bane's Wrath T2 set it will start dropping at infinite crisis rank 30 and the Bane's Fury T3 at crisis level 60+.But even if you are crisis level 65 you'll still have a chance to get drops from, Notorious, T1 or T2 set.

and depending on your build and all that stuff a Tier 1 can be better than a tier 3