Highlights for me were the coastal plants, living as I do near the Cairngorms I don't often see them.
I suppose the real gem to look out for is the Hebridean Marsh Orchid, Dactylorhiza ebudensis, which is found in one location in North Uist, again the time is right for that.
There are quite a few knowledgeable people who post on the Wild About Britain website forums, I know there was at least one person who spent a few weeks on the Uists last year, it may be worthwhile asking on there.
I'm going to the Isle of Tiree in late June this year, and have a very good book, 'Wildflowers of Coll and Tiree'. I know that geologically they are very similar to the Outer Hebrides, mainly Lewisian Gneiss, overlaid by peat in the centre of the islands with machair on the outside, it may be worth having a look at that.