I'm not having any trouble with downloading files, but it keeps arbitrarily replacing filename information with numbers. What is that and how can I stop it?

That might happens if you are downloading a file from two host and the hosts named the file differently. LimeWire itself doesn't add anything to the file name, it uses the name supplied by one of the hosts it downloaded the file from.

Macs limit the amount of characters for a file name. So, if it's a long name, it will add letters/numbers to it. Such as: "A very long title name.mp3" will become: "A very lon#B1843.mp3" or something like that. It's not a problem. I think it's "code" so that the full title will come up when others search for it.