For base layers I think you can get by with many different options. Merino wool is nice, UA is nice also, but for me they aren't totally necessary. I've actually always thought the UA stuff was incredibly overrated. I buy the Rocky base layers at Walmart at the end of the year when they go on clearance for $5. They are plenty warm and very comfortable. To me, the biggest thing like Milo said is to have a windproof outer garment. If you're letting wind get in you're not going to stay warm, no matter how many layers are underneath. So my strategy is, buy base layers cheap (so I have a lot of them and can swap often) but buy high quality outerwear. My Predator fleece keeps me incredibly warm for as lightweight as it is. And the fleece vest by Predator is a great addition too when it's really friggin cold.