If you're going to forge flat stock into knives, 125 lb+.....if you're planning to forge
round other thick stock, then you'd be better off with larger.
How you mount it is as important as what you use. The base should not allow
the anvil to walk or bounce.
Brand.....I prefer older ones. Hay Budden, Peter Wright are very good old
anvils if they're in good shape. Try the well known auction site under blacksmith
anvils. Good older ones go for around $3 + per lb. newer ones cast steel not cast iron,
may be 2x that.
I'd recommend taking your time..also it's best if you can go see the anvil,
and take a ball bearing with you. Drop the bearing on the face and watch
rebound....90-95% is best.
Also....try Craig's list in your area...