I'm having the worst time locating a good-sized anvil at anything approaching a reasonable price, but I did manage to find an 80# Fisher with a broken heel. I've heard that proper mounting on a good stand can help offset the lack of mass. My best idea so far is a length of I-beam concreted in the ground a couple of feet with a mounting plate welded on the end. Would that work out ok, or do I need to come up with something else?

Seems sturdy, with anything anvil related its the thicker the better, cram as much mass as you can under the hammer. Sounds like it would be well suited for that size anvil if you want to put the time into it and you would still be able to use it if you get a little larger anvil. I would want some type of bracket to be able to lock the anvil down to the base plate as well so it wont wobble.

Wood bases can be ok as well, lots of guys build them or use tree stumps for their anvils and they work great. I made one for my rail anvil.

this thread has some very good ones in it and could be worth a look as far as inspiration goes