MP3's are usually stated as a bit rate, so a 128kbps file is "kilo-bits per second". I don't know if one could extrapolate an equivalent bit depth and sample rate from that.
When capturing the processed version stay at 24-bit. If the client demands an MP3 as the return format then send him the best you can, usually 320kbps. I make my MP3's directly from the 24-bit files, some people make them from the 16-bit versions.