For what you want to do you should first try a pneumatic planishing hammer. Harbor Freight sells a cheap one for about a hundred bucks.
http://www.harborfreight.com/pneumatic- ... 94847.html They do about 1000 -2000 hits per minute and have several interchangeable hammer heads for different size impressions. I built one from a cheap air hammer with a modified hammer head and some 1-1/4 pipe for about $30. Then I graduated to a CP sytle hammer once I saw how much better they were. But I still have and use my original home built hammer and a HF plashing hammer plus a couple mechanical hammers too. A boy never has enough toys or hammers...
If that isn't fast enough for you then you can add a CNC X/Y table later. For sheet metal work, normally the metal is held by grippers on the X axis gantry and the hammer is stationary. My big mechanical hammer weighs between two and three thousand pounds and has a 48 inch throat. The X/Y table has to be fairly heavy too to stand up to the pounding that the sheet will receive.