So you decided on how you are going to heat up your metal, but what are you going to pound it on? There is a little bit to consider before you go getting the hunk of steel you'll be practicing on and making your masterpiece.
What do you plan on making? If you are just getting started, chances are slim you are going to be making some high quality items. No offense, but good blacksmiths don't happen over night. In all likelihood, as proud of your work as you may be, the first few times you make something it will be pretty crappy. Having the proper anvil weight will reduce the stress you'll face while trying your hardest trying to make something worthwhile. Generally speaking, heavier anvils for larger projects, lighter anvils for smaller projects.
If all you want to do is fulfill that childhood fantasy of making knives, you don't need some 200+ pound anvil. When it comes to anvil weight, heavier anvils effectively let you shift more metal with each hammer blow. This is because, essentially, the heavier anvils have higher intertia and resist any sort of movement. Since the force of your hammer strike needs to go somewhere, more of it goes into the metal you are trying to shape. From the sounds of that, it would seem that you want as heavy an anvil as you can get. After all, faster changes, fewer strikes, what is wrong with that?
The problem arises when you are trying to do something a little more delicate, say, trying to finish an edge on a blade, or doing some work on small pieces. Unless you have a collection of hammers and a good amount of skill, a too-heavy anvil will just lead to you spending almost as much time correcting overstrikes and dents as you spend actually properly shaping your project. In short, bigger is not necessarily better.
When it comes to smaller anvils, there are a lot of do-it-yourself attempts out there that have various levels of effectiveness. A bit a railroad rail, for instance, can have a decent surface and spring to it, but shouldn't be a long term solution. A properly made anvil will ring with a nice pitch and your hammer will bounce back with a nice spring quality. It shouldn't be a dull thud with no life to it.
If you are just getting started, my opinion is that you may be best off with an anvil between 100 and 150 pounds. This will give you pleanty to work with as far as size of the face (top part) of the anvil, will be heavy enough for rapid enough shifting for you to develop your hammer skills without worrying about shifting too much metal, and, in a similar vein, it is heavy enough that you won't be pounding away forever hoping for the metal to change shape.
Like with everything I talk about, make sure what you choose is within your budget. Nothing wrong with using an unconventional anvil until you save up the money for the real deal. As of late, anvils have been running about $4+ per pound. Blame the anvil collectors, they exist, and they have taken a lot of usable quality anvils off the market to gather dust somewhere. That is a tirade for another time.