Another option would be U-232, which could suit your needs?
It's free, open source, secure and is constantly being updated by professional coders and uses memcache so you should have no issues with large amounts of peers? V4 is currently in the works and has XBT built-in and will have an option to use the php announce or XBT announce.
I suggest you check them out if you haven't done so already before you make any decisions on a new source or spend any more money!!
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Anyone can suggest a source code to use, though most of the time it's a biased opinion, so I'd suggest you look/try the various source codes that are about and make up your own mind to see if they suit your needs.