Among the things I would suggest as essential are:
Add to this the usual traveller’s staples such as toilet paper, a flashlight, a knife and a basic first-aid kit, plus money (in a money belt or neck pouch hidden inside your clothes) and anything you need to record the trip – camera, film, sketch pads and notebooks etc.
If you’re going on an organized tour, the tour operators you can hire porters to carry them.
Everything you don’t need should be left behind. Many hostels and hotels in Cuzco will let you leave stuff with them. Your pack will already be uncomfortably heavy with just the essentials. (Actually you can leave your stuff at your tour operator storage)
Remember the dramatic temperature range. Irrespective of what the weather forecast says, you’ll want clothes for both warm and cold weather. At low altitudes it can be quite warm; on the peaks and at night it will certainly be very cold. The best thing is to work on the layer principle; if it gets too hot, take something off, if it gets too cold, put something on. A waterproof and windproof jacket is also essential.
Take strong, comfortable footwear. Heavy boots are probably not required or advised; all you need is something that you can walk in all day, that gives good traction and which supports the ankle. Lightweight modern hiking boots are probably ideal. I wore a pair of extremely cheap rubber-soled canvas boots from an army surplus store. They had the advantage of comfort and light weight, but offered little traction and no support. They were also not very robust; I survived the Trail; the boots did not.
Remember to wear your boots in thoroughly before you go and get used to walking in them. Blisters will reduce your enjoyment significantly.
Yes. There are shops in Cuzco which will rent or sell equipment. However, bear in mind that shops may not have everything you want and that the stuff they offer to rent may be old, broken, heavy or have parts missing. Check everything before you leave the store.
It should generally be possible to fill your water bottle from streams and rivers along the Trail. You must use sterilizing tablets or boil the water (remember that water boils at lower temperatures at high altitude, so you must boil drinking water longer to ensure its fully sterilized). Take water from streams in preference to standing water, and filter it if in doubt. Be careful when taking water from fast-flowing rivers; by inconsiderately falling in and drowning you risk polluting the water supply for everyone else. (Actually your company will supply boiled water every day).
Photographs and all your rubbish.