A 50lb anvil will work for your purpose. It's not about the anvil weight, it is about how the anvil is fasttened. If you want the anvil to work well, it cannot move on the stand, and the stand cannot more From the earth. If you use a stump as a Stand, forge at least four long tacks for to hold the anvil down. The stump must also be buried in the ground ( I have heard at least 3 feet). You can also make a steel stand but the same rules apply.