Let me first state that I've never forged a knife, so take my comments accordingly. I bought a lot of three anvils from an estate sale, all around 100lbs. One of them was a Vulcan. I ended up selling off two of the three as I didn't need three anvils, and probably don't even need the one I kept yet, but I might build a forge some day and a man can never have too many tools right?
The Vulcan anvils that I found while researching are a little different than most anvils in that they are made of a wrought or cast iron body with a face of hardened tool steel bonded to the body. Most anvils are solid cast steel. Because the anvil is made of two different materials bonded together they tend to be very dead sounding when struck. The anvil doesn't ring like a solid steel anvil. This can be a very desireable quality if your neighbors are at all sensitive to the sound of you pounding on an anvil. Unfortunately in my case, the Vulcan was pretty beat up. They are known to be prone to chipping and the one I had was pretty badly chipped around the edges or I'd have kept it because if the lack of ringing. It had good bounce, a near 1" steel ball bearing would bounce back about twenty inches when dropped from a height of two feet so the desireable rebound was good. You might want to get a large bearing and a yardstick to check rebound this way before purchasing.
From what all the real smiths tell me, 100lbs is a bit small for most, but is a decent sized starter anvil, especially if the price is good.