Cast steel is usually fine. Face hardness should be stated.
For knifemaking an anvil of 60-120# is all you need. You can make a solid base from wood or pout concrete into a washtub and add more mass below the anvil.
A good ratio of hammer mass (force) to anvil mass ( resistance) is around 1:25. A little more is good, so 1:30 is enough. 1:60 is maxed out and you gain little beyond that. What does all that mean? Using a 2#/1.5kg hammer, the anvil must weight no less than 50 pounds, with 60 pounds being a better lower range. At 120 pounds, you have more than enough resistance from the anvil to the force of the hammer for any normal blade forging needs. A 400 pound anvil may make others jealous, but it won't forge appreciably better than a 120# anvil.
What is far more important in forging is having the steel at the proper temperature. Most new smiths forge way too cold. This is just wasting arm energy, as the steel won't move under the hammer much at all until it is above 1700F. If it gets above 2200F, it may start to crack or crumble in regular forging. The best forging range for most carbon steel is 1800F to 2100F.