IMO, what you should look for is:
Size - bigger/heavier is better, buy as large as you can afford. You've heard the old adage: Pay once, cry once.
Surface - Smooth (no nicks or dings), Flat and hard is ideal. a lot of folks will take a ball bearing and drop it on the anvil from a couple feet and it should rebound pretty well. If the ball bearing doesn't rebound much, the anvil is considered "dead" and not worth your cash. You want the hammer blow's force transferred to the piece, not get absorbed by the anvil.
If you could swing another $800-ish I'd personally buy (waiting for mine, actually) a #260 Nimba Centurion.
I guess the first question should have been for what type of forging? If it's just knife making, then you don't necessarily need as large a face as the Nimba. Absolutely wonderful for general forging though.
Good luck
~billyO