Here are a few things that are nice to have but not imperative to starting out.
sand bag- I currently use one and have found that it helps with the noise.
painters tape- So this isn't totally necessary, but I have found that it helps a lot. You don't need the "straight tape" or anything fancy. Masking tape or blue painters tape work just fine. Helps keep lines straight and keep your piece from moving at all while on the bench block.
design stamps- Again, this will depend on what direction you are going with metal stamping. Michael's carries a few designs, and so do many of the websites I will be listing in the "Helpful Sites" tab. Etsy is also a great place to find them :) Usually they run around $8-$15 depending on size and how intricate the design is.
dapping block- This is for making your blanks domed. Something I have yet to get into :)
ball pein hammer- Used for creating texture on your metal pieces.
disc cutter- This is if you wanted to cut your own blanks from sheets of metal. Eventually I'd like to get into this, but not for awhile (will do a post on this later).
Files- Not necessary unless you are cutting your own blanks or get blanks that haven't been deburred. Deburring is the process in which the rough edges of the blank are removed either by hand or with a machine. For beginners I recommend that you purchase deburred blanks.