Any question answered...

And what info do I need before buying one?

Steerer length is the critical fit dimension, axle to crown length should be pretty close to what you're replacing if you don't actually want to change the geometry of the whole bike, and fork offset should match what you had before or you'll change the steering geometry, although that's not all that critical unless you're already on the edge of the envelope.

Really though, you probably shouldn't even be replacing a threaded fork, threadless is superior so take the opportunity to upgrade with a new headset and stem.