Are cached things 2/2 colourless creatures while in the cache? If not, that could be confusing.

Not that it's that important for this particular one - the ability to do it at instant speed is worth the extra cost.

Only face-down cards on the battlefield are 2/2 creatures. Cached cards are exiled. And technically, if cache works like morph (I assume it does), it's actually even faster than instant.

Cache occurs at instant speed, and casting the spell from the cache occurs at the normal speed for that spell. The only part of Morph that occurs as a special action is turning the card face up for its Morph cost, if I understand Morph correctly. I should probably reword cache slightly to make it sound less similar to Morph.
Cache is actually a sort of cost reduction ability. It lets you spread the cost of a spell out over multiple turns by giving a "down payment" of . It costs more overall but lets you keep more mana open each turn.
EDIT: I changed the name of "cache" to "ready."

I updated Prepare to Foretell on the mechanics page, which turned this into a better Doomskar. I hit really close with this, though; I planted for prepare to cost and Foretell costs , so the costs were very roughly equivalent.