Would 2nd Usinish bothy as a great place to stop.
When I camped on the west coast of S Uist by the beach, I noted that finding fresh water was a problem. There are a few streams, but they are slow moving and run across fields with animals in them, so I did not fancy using them. There are many houses dotted about to ask for water, but many of them looked un-occupied.
If you do go to Lochboisdale, the Hotel (nice lounge and meals) lets you camp behind for free, and there are showers in the village to use.
Barra is magical, if you stop in Castlebay, go for a curry in cafe kisimul, but book well ahead.
EDIT Just noticed this is an old thread, but my info may help someone else.
Edited: 18/07/2012 at 09:49