My character is a spellthief and it has no access to level 0 spells. I'd like to get some levels as an Arcane Trickster, but I only have access to Greater Mage Hand, could it be enough to qualify for the prestige class?
Technically, no, mage hand and greater mage hand are separate spells and knowing greater mage hand doesn’t count as knowing mage hand. You cannot qualify for arcane trickster with greater mage hand.
But the only reason that’s true is because arcane trickster was written before greater mage hand existed. Ask your DM—I’d be surprised if they had a problem with it.
But note that arcane trickster isn’t a very good choice for a spellthief—you already get sneak attack, which is the main advantage of arcane trickster, and arcane trickster won’t do anything for your spell stealing. Ranged legerdemain is cool and all, but you can do much more with spellthief than you can with arcane trickster.