So I was wondering, are there any must get items in the cash shop? Like I know there's the exp boosting stuff, and those 90 day buffs, and inventory, but just how much extra convince do they really give? Also, if I really should get some of them, how many of each should I get and how much would it end up costing me?

Those boosts are best preferred on the current end-game areas, such as Pipe3. Its best preferred to save your OC till later.

Ok, but what about things like the inventory upgrades?

Dont really need, just keep givong friendship items to Shizuka and your inventory space will expand.

As long as you don't hoard items then you don't need inventory space. The only item that may or may not be worth getting (when you're in need of them) are the buttons. (In my opinion of course), and that's only if you have a high lvl weapon that would be worth +100ing.

I personally advise getting inventory space expansions...

This is my inventory - without any unidentified weapons, and with very few identified ones.


(It's a screenshot of the storage menu, left is Shizuka's storage, right is my actual inventory)