I鈥檓 inclined to say that you’ll want a pair of external gaiters. The Arc鈥橳eryx Alpha SL pants ($198) are great, but I’m not sure you’ll be comfortable wearing waterproof shell pants all day. Plus, they’re made of Gore Paclite, which is a wonderful material but isn’t the most durable stuff on the planet.
If I were you, I would carry the Alphas and wear some light insulated tights and shorts when it isn’t raining. You might also look into Arc鈥橳eryx鈥檚 excellent Gamma LT pants ($169), which are made of a light softshell that is rugged, comfortable, and water repellent.
For gaiters, Outdoor Research’s Salamanders ($45) are a good choice. They鈥檙e a short gaiter made from a waterproof-breathable material, combined with Cordura in the lower section for excellent durability. When hiking off trail, the foliage is apt to be wet even if it鈥檚 not raining. That moisture can go right into your boot as you kick through shrubs.
If you expect a really wet trip, then look into a full-length gaiter, something like the Mountain Equipment Co-Op Kokanee Gaiter ($39, Canadian). They’re made with Gore-Tex uppers and tough nylon lowers鈥攋ust the thing for extended hiking in wet conditions.
Have fun!