Can someone suggest what the traffic limits should be? There doesn't appear to be a user manual. I often notice that if I add additional links to the download list, the download speed from the previously added link is reduced in speed. I'm not sure if that's a result from the server itself or not?

It depends on your hardware and on your Internet connection. For the start you can read older messages:
search.php?keywords=bandwidth&terms=all&author=Usher&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=-1&t=0&submit=Search