For the second question, you might be able to get away with using a random one or the one set in the code, but with more than one 'duino, it causes more problems.
On the shield box or on a sticker on the shield, you should find the MAC address that you can use.
For the third question...it's a bit hard to say. Some might be compatible with your model, others might not. It's worth asking the seller about compatibility, but do be aware that eBay can be a little hit-and-miss with buying parts.