Those "random" parts downloaded, aren't that random. it has to do with piece size. The client will download whole pieces. Let's say piece size on the torrent is 1MB. The file you want from that torrent is 41.1MB. If this is the first file then 42MB will be downloaded, where 0.9MB will be from the second file. I.e you will get a 0.9MB partial file.

I don't know of any client, which downloads and seeds a partial piece.

BTW both Vuze and uTorrent have the option of accessing and prioritizing/weeding out specific files from the file list after the torrent has been added, as well as upon adding the torrent. So why you even bother using a joke torrent client goes beyond me.