Layer. After that, it depends on your budget and personal preference. Things like Gore-tex are very nice and some people will pay extra for them, but they aren't for everyone. I would try things in store to ensure you like them before buying.
Layering is everything, especially if you're going to be walking/ hiking. And try on everything and layer it how you're intending to wear it. Wasn't use to layering and felt kinda constrained on my trip to Banff. Clothes feels tighter the more you layer, weather ranged from 20-30 during the day, and dropped to 10 or lower at night. But with 3 layers it was sufficient, take into account how much you'll be moving around. Think ice Iceland stays around 30-40 around that time. Preference- base layer North face ( Come in 3 different thickness so you can choose, breathable and comfy, not itchy), mid layer fleece pants and jacket Columbia, then had Columbia snow pants ( dont think you'll need snow pants). Got a Patagonia thin under/mid layer but the inside stitching was rough def felt it rub against when hiking. But did get some Patagonia shirts and they were comfy. Arcteryx is really good. Columbia has done me well as a breathable shell, some shells aren't breathable
Depends on what you are willing to spend, and if you want it to last long-term. They have loads of great options. Outdoor research, Arcteryx, Kuhl, etc. Fit might be another thing to consider. Cut is different for each of the great brands.
My favorite brands are Mountain Hardware or Patagonia. My suggestion is that you get a good base layer. (Under Armour baselayer 2 is my favorite) and a lightweight gore-tex shell. Pants and jacket. Then, just remember to layer, layer, layer!
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