Why would you care to exclude peers slow or far? What benefit would you expect?

The torrent library to use is libtorrent.

https://github.com/rakshasa/libtorrent

S3 will normally provide ample bandwidth, but in some situations there's a local shared downlink that is the bottleneck. For example in asset heavy multiuser VR or game apps used by many people in same location. By only including the nearby peers I get all the benefit but minimize the risk of people accusing me of clogging / mooching off their uplink.

(You probably want Arvid's libtorrent, at http://www.libtorrent.org, not the one linked above.)

There's webtorrent by feross! https://github.com/feross/webtorrent