General

Level 60 would be your restriction on both cubes and special scrolls (Guardian, Safety, Shield, etc.)

Here's a full list of Maplestory's restrictions:

MapleStory

-Potential-related items, including Miracle Cubes (all kinds), Miracle Circulator, Tim's Secret Lab, Awakening Stamps, and Potential Slot Carvers cannot be purchased with NX Credit or Maple Points until a character has reached level 60.
-Special scrolls, including Shielding Ward, Shield Scroll, Guardian Scroll, Perfect Innocence Scroll, and Return Scroll cannot be purchased with NX Credit or Maple Points until a character has reached level 60.
-Messengers, including Megaphones (all kinds), Messengers (pop up), and Maple TV messengers cannot be purchased with NX Credit or Maple Points until a character has reached level 60.
-Cash Shop Surprise items, including Premium Surprise Style Box, Holiday Surprise, and Halloween Surprise cannot be purchased with NX Credit or Maple Points until a character has reached level 60.
-Vicious' Hammers, Meso sacks, and Gachapon tickets cannot be purchased with NX Credit or Maple Points until a character has reached level 60.
-Special Gachapon items cannot be purchased with Maple Points until a character has reached level 60, and cannot be purchased at all with NX Credit.
-Marvel Machine Spins and Double Marvels available on the official MapleStory website cannot be purchased with NX Credit or Maple Points.

I'd recommend buying Style boxes

I don't know.