For the 70, 20, 10 chance, you could do the following: create a local variable (for this example I'll call it ItemClassSelector).
In the event you currently have, set ItemClassSelector to int(random(1,11))
Then use 3 sub event, 1: if ItemClassSelector <=7 then pick from common ; if <=9 & >7 pick from rare ; if =10 pick legendary.
For the animation selection use more sub events. If common set animation to "common", If rare set animation to "rare", If legendary set animation to "legendary".
Nothing will stay unlocked until you start saving, here is a tutorial:
https://www.scirra.com/tutorials/1461/how-to-use-the-localstorage-plugin
I assure you saving will seem really wired and complicated at first but it is really simple once you get a feel for it.
The data structure you will want to use to save variables is a dictionary.