I'd recommend MediaMonkey (
http://www.mediamonkey.com/). Great all-around music management program that works well with my Sony player (an NWZ-A818) and should work just as well with your Sansa player.
There's both a free version and a paid version of the program (the paid version has features not available with the free version). Based on your post, one feature of the paid version that you might be interested in is auto conversion. You can set MediaMonkey to automatically convert some or all files as they are loaded on your player (it doesn't affect the files on your hard drive).
As an example, my player doesn't support MP3s with a bitrate lower than 32kbps. Since some files, such as podcasts, have a low bitrate, I have it autoconvert any MP3 with a bitrate lower than 64kbps to 64kbps (I prefer that as a minimum bitrate for audio regardless of the type of program).