My recommends are as if I were building such a structure for myself.
Post bottoms and all cross grain cuts sealed immediately after cutting. Post bottoms are further protected by ice dam rubber (bitchithane) and they sit on short piers so water cannot pool around the posts.
You can use a quality wood deck water sealant on all exposed surfaces applying two coats (Olympic would be my choice). Spray and/or roll it on. An alternative would be water based external polyurethane.
Use only quality coated metal fasteners.
expensive but worth it use composite windows from Anderson.
Your cost to build just went up.
Good luck.
