I have large and small roll top panniers (100% waterproof)and a spare rack if anyone wants to borrow as well as a medium carradice saddlebag. I can stick them in the post if thats easiest to get them to someone
For what your proposing I'd personally go for either 1 x large pannier or two small panniers. However a large waterproof stuff bag bungeed to the rack would probably be enough.
Be minimalist, think twice about every item, you'd be surprised how little you really need.
When I toured it was a case of if it couldnt go in 2 large panniers then it was too much, though that was cycle camping for at least a week.
What you wear to ride, rinsed out and dried at youth hostel for return leg and a set of civvies for Ullapool would be clothes contents. Only tools you need and essential spares. Why bring beer/food, surely that exists in Ullapool, trail rations only.
Ultimately everything you carry is weight to drag you down so dont bring it unless its essential