A midsize waterproof pack with a belt for weight distribution, breathable back panel and lots of pockets is best for trekking. Super lightweight packs will modestly trim your load, but test a range of options at a large sporting goods store like REI. Internal frame packs are easier to balance, riding closer to your back, but they discourage ventilation. A volume of 3,500 cubic inches is ample for the 35 pounds or less you should be hauling.
Footwear must be waterproof, tough, ankle-supporting, as light as possible and used. On Nepal's trails, boots will not be broken in--you will. Leather-fabric hybrid boots are lighter and more comfortable than all-leather, but much less waterproof. For evening footwear, Sketchers, Crocks or running shoes are comfy, durable and light.
Season of the year dictates the temperatures you will encounter, but a bag rated for 10 to 20 degrees below zero is siding with caution. At 15,000 to 18,000 feet, sub-zero night temperatures are common in October.
The layering model is ideal---a warm, moisture-wicking base layer like polypropylene, a mid-layer of fleece and a veneer of windproof, waterproof, breathable fabric. A wool sweater makes a lightweight concession to lower temperatures. In winter months, a down or fiber-fill jacket or parka is advisable.
Nylon pants are fine until temperatures plummet. At that point, wicking long johns and breathable rain pants should take over. You will need two hats---a wide-brim model for sun protection and a wool cap for evening warmth and sleeping. The dorms and rooms available are not heated. Take a pair of shorts, light gloves, a couple of T-shirts and two pairs each of light synthetic and heavy wool socks.
Trekking poles save knees and offer balance points. When you don't need them (or need your hands free), the telescoping variety can vanish into your pack.
Other necessities are sunglasses, maps, a headlamp, paperback book, small pocket knife, sunscreen, mole skin, bandaids, toilet paper, antibiotic cream, chapstick, lightweight digital camera and binoculars, as well as gaiters for snow walks.
Low humidity, faster breathing and more sweating mean big-time water loss at high altitude. Three liters per day (at 6.7 pounds) is not an unrealistic water-intake goal, but you can often resupply at teahouses en route. If you refill from streams or lakes, be sure to have iodine or chlorine purification pills. All manner of elaborate hydration backpacks are available at sports stores.