I don't think heavy snow will e an issue on the islands, more likely heavy rain and strong winds in the winter. I can't guarantee what it'll be like on the mainland, though. I don't know of any stake-outs for notable species that are on that route. (I assume you're taking the A82 to Fort William and going that way). There may be chances for Crested Tit, but I think they're more regular around Speyside. There are a couple of sites for eagles on Skye, but you've got a much better chance on the Uists. Any of the sheltered bays on the west coast may hols divers and it's always worth keeping an eye out over the moors. Loch Lomond may be worth a quick visit, there's a "Loch Lomond / Galloway" thread here on BF, it may be worth asking folks on there for up-to-date info.
I'm sure you'll have a great time up there. It's very undisturbed, any birders you bump into are likely to be friendly and helpful and it's very easy to get around.
Cheers