If by laminated, you mean that they are forged from several pieces, than you are correct. Forging several pieces together to make an anvil is very common, and not something to worry about.
John K pretty much covered usefulness. Make sure it is flat. Make sure the anvil is all there (not missing the heel or half a horn). Make sure someone didn't take a chisel to the face \ horn, leaving cut marks all over it.
The weight can not simply be "5", there must be other markings as well (maybe faded away though). It will need 3 numbers, such as 0 . 5 . 0. That being said, that is an impossible number, as the center number represents quarters. If it is the middle number (assuming the anvil would be massive if the 5 is on the left, and miniature if it was on the right), it has to be a 1, 2, or 3. Likely a 2 or 3, making it ~55-85lbs.
One simple thing you should do if it passes the flat\not missing anything inspections, is bring a hammer. Gently strike the face and horn of the anvil. Being that it is a forged iron anvil, it will not have a sharp ping, but you will still be able to tell if something is wrong with it. An anvil with a crack in it will not have any ring at all when struck, and will make somewhat of a flat, clank noise (sorry, that isn't very descriptive, but you'll know if it sounds broken).
Also consider the style of anvil and if that is what you want. William Foster's are all older- English style anvils, thick waisted but sturdy. Personally, I love the English style, but more common is the more modern "London" style, which gives a larger anvil for the weight, and greater ability to work around the heel and horn.
As far as price, William Foster is a somewhat uncommon anvil, so there is some collector value there if it is in good shape. John K is pretty spot on with $3lb for a nice anvil. If it is really around 85lb though, I'd shoot for under $200.