KUIU is made for the "athletic" build, meaning not much in the way of body fat...the insurance chart weights. So, you will normally need a size larger, but not always. Check out their sizing chart to see what would fit best for you. Try on your nephews clothing if he is close to you in size, or use his sizes as a starting point.
As for what to get, most people use the layering system approach of:
~ A base layer of merino wool, you choose the weight (185, 2xx, etc.) depending on the weather you anticipate. This layer is to wick moisture from your body and to help keep you from getting too cool when not moving too much.
~ An insulating layer to add warmth to your overall system. Most people these days go with a puffy jacket in either a synthetic or down insulation. You can use whatever you want though...a wool sweater, puffy, insulated jacket, etc. You might want to get the same kind of garment for your legs if it will be really cold or if you plan on sitting still for long periods of time.
~ A wind and water resistant outer layer to go over the previous two layers. This can be a softshell material like a guide jacket and attack pants or breathable rain gear.
~ A final outer layer of rain gear, if you don't use it for your third layer, that protect you from heavy rain and winds while keeping in your heat (and maybe moisture too, so you need to choose wisely about when you wear it and what yo are doing).
KUIU and other brands of technical clothing like First Lite, Kryptek, and Sitka Gear all use expensive materials for their clothing which is why the price tag is so much higher than just getting Carharts or the generic brands from Bass Pro or Cabelas. The fabrics are worth it though if you are facing rough weather or conditions. One of KUIUs focuses in their product design is to make it a lightweight (not flimsy) as possible so if weight is not an issue for you then you might be able to find a "good enough" item with another brand that is a bit less.
Like mtnclimber said, many of us wear a mix of different brands since we picked them up over time or went for the best bang for the buck. You can find some good deals on sales at the names listed above. I always check out Cabelas sales and bargain cave...I bought my current puffy jacket there and it cost me less than $100.
Hope this helps,
Larry Schwartz