Just about any substantial piece of steel makes a usable anvil. The more substantial, the more usable. Mild steel will work for an awful lot of work and it will take years to wear it out. Harder steel (forklift forks, I imagine) would do yeoman's service, but probably best if they're attached to some other large hunk of steel, though sitting in a bed of sand could work too.
From your question, I assume that you're an amateur blacksmith (as am I, though I have a few years of dabbling under my belt). If you have a hard surface that holds pretty-much still, you've got all you need in an anvil to get started. I'm not speculating - I have used both a bit of steel sitting on the ground as well as the ~#500 lb anvil I lucked into some years ago. To a duffer, there is little difference.
Perhaps someday I'll be good enough to appreciate the awesome anvil that I have, but I can assure you that starting out it doesn't make a difference.
GsT