You can't easily know if someone is leeching, since that host may just be busy. There isn't an easy way to block leechers with LimeWire.

http://www.bearshare.com/stats/ collects quite a bit of info on their part of the gnutella network. Perhaps it will give you some idea of the leech % and average number of shared files per node. 350 sounds OK

http://www.videolan.org/ offers a free player that works well and often plays media files others can't. Maybe it will play the file properly, or at least tell you if part of the file is missing something (there are a lot of corrupt media files out there).