Ok so baselayers tend to come in lightweight and medium weight wool. The lighter weight is better for all year round, but less durable. The medium weight is better for colder months. And more durable because more material.

I have the Icebreaker 250 medium weight base layer and at first since I never wore wool before I found it slightly itchy but over time with washing I don’t notice it anymore. I recommend it for colored seasons.

I think a gentle detergent without enzymes that break down protein (like wool) is a good idea. Some recommend detergents with lanolin for softening, but others say the water resistance that lanolin gives is opposite what you want in a base layer, as you want the wool to absorb the sweat. I haven’t really tried either to be honest! But something to think about.

If you don’t mind used, Icebreaker is easier to find cheaper. Smartwool tends to be priced high even used, but it might keep its value better as it’s a known brand.

Merino base layers usually do fine in the washing machine. But hand washing probably does give the longest life. Especially if you have a washing machine that has an agitator like a top loading washing machine which can be harder on delicates. If so I recommend a delicates bag and wash on delicate cycle or wool cycle if there is one.