Alpacas have relatively simple needs, shelter being of utmost importance.
Although simplicity is the rule when starting out, many alpaca owners expand shelters to encompass multiple care needs such as vet rooms, shearing stations and breeding pens. Don't forget hay and feed storage, it's especially important to keep these items close to the alpacas for your convenience, though protected so as they can't get into them.
A basic 3 sided structure with it's back to the prevailing wind, siding that comes completely to the ground, and a non-leaking roof is required. A 12 x 12 structure will suffice for 3-4 alpacas.
Either wood or metal siding works fine. A metal or composition roof that'll keep the rain out and hold up to the snow loads is also important. Be sure to check for and eliminate any sharp or protruding surfaces that can harm your alpacas.
Provide a rubber or non porous surface to cover the flooring to ensure your alpacas have a dry area to bed down.
When deciding on a location for your shelter, keep in mind the direction of water runoff during the rainy months to avoid having a wet floor during the winter. It's nice to provide straw bedding especially during the cold months, but not necessary.
Alpacas enjoy being together at all times. Even though at times its necessary to keep them apart such as during quarantine, illness, postpartum, you'll find that life is simpler if everything is under one roof, including the alpacas. Granted, you will have to employ the assistance of panels or fencing to keep them physically apart for various reasons, but if they can see each other, they are happy and content.