When I was thinking about what would be the most important item in Iceland, I got to two things: a good, waterproof and warm jacket (ideally with two layers, like 2-in-1) and waterproof boots.
Amazingly enough, I found exactly a 2-in-1 Columbia jacket. It it basically one warm jacket which zips unto another one - waterproof and thinner layer with a membrane. I have been using it all winter now, and must say, it is amazing. When it´s freezing outside without any chance of rain, I put on the warm layzer and leave the outer one at home. When it rains, but is quite warm outside, the outer layer has its go. It´s amazing.
I also got very nice shoes from Grisport, all leather. These were the cheapest possible version of a waterproof, strong hiking boots I was looking for. Weren´t that cheap, but I was satisfied anyway. I have been trying them as often as possible when going on little trips around Brno. I got a model that is one size bigger than my actual size, because of a need to put on thick socks in some situations. Other than this, the shoes did not prove a bad investment. They are great :)