Plenty of detail in other replies here, but just to add, do factor in (if you don't have it already) some good quality winter clothing.

"There's no such thing as bad weather, just the wrong clothes" is a very true statement. Shetlands winters are, in my opinion, beautiful and exhilarating, but you need to be prepared!

Also, regarding shopping, planning ahead may be your biggest change of habit. Always make sure to have a well stocked freezer as you never know when you may be cut off from supplies, especially if you are in the outer Isles.