The autumn is the season of metal element. It is a very  effective season to use coin for Fengshui purpose. Many Fengshui practitioner believe the most effective is the Han dynasty coin. The next to the best is Tang dynasty coin. Then it is the Qing dynasty coin. However, those coin might not be difficult to find in antique stores in China or east Asia, they are very difficult to find in the west. For beginners use, I would suggest to start with reproductions that you can easily find in China town or on the web. 
To select coin (reproduction), one critical issue is the copper content. A  good coin should contain bout 95% of copper in weight. Less copper content means lower quality. Needless to say, avoid wood, clay, plastic or other material  reproductions. 
The size of the reproduction is constantly in debate. Some believe that larger is better. Some believes the reproduction should  reflect the actual size of the original.  I personally agree with the second opinion.
The coin should be hang on yellow silk or gold chain. The color that helps metal element. It is important to notice that color red has character of fire element.