I have a C700, the only thing I've bought for it is a filter holder (plus UV and polarizer filters). I can't imagine needing a telephoto with this camera...
I was looking for an external flash for it but decided it isn't worth it - a couple hundred for the flash (if I get the one that's designed to work with the camera), plus $50 or so for the cable, plus another $50 or so for the bracket...