An avil is the heart of the Bladesmith's shop. Taking your time, and finding a "good" anvil will pay benefits for the rest of your career. As has been mentioned, stay away from the cast iron "el-cheapo" anvils. They will become swaybacked in short order. The anvils with aluminum bases were designed specifically for light duty, and are built that way to be portable for farriers.
My personal favorites are the anvil that have a wrought iron base, with hardened face plate (Trenton brand is the one I like the most) (NOT CAST IRON BASE)...there is a huge difference. For new anvils, I have worked on Nimba, Old World, and Emerson anvils. These are all cast steel anvils, and all a very servicable.
Testing an anvil with a hardened ball bearing is a very reliable test to determine the "working ability" of an anvil. Hold the ball bearing approx. 1 foot above the face and drop it. The ball bearing should return to the level it was dropped from, or higher. This indicates that the anvil will return the energy from a hammer blow to your work piece, effectivley reducing the force required on your behalf to achieve a given amount of work.
The general rule on anvil size is no less than 10 lbs of anvil for every 1 pound of hammer weight. That being said, I consider 125lb to the the minimum necessary for a Bladesmithing anvil.
I recently had a student here who found an anvil locally, and purchased it. He took it back to Texas with him, and is having the face surface ground (had some slight roughness to on the face) but was otherwise an outstanding anvil. Even with the machining being done, the cost was around $2 per pound....buy the way, it was 176 pound anvil.
Take your time, look around, and make an informed decision when purchasing an anvil. With minimial care, a good anvil will not only last your lifetime, but will be there well past your grandchildren's lifetime.