I'm a big fan of those water bottles with built-in filters. They're very easy to use and effective if you follow the instructions - you simply fill up at any village tap and have rapidly filtered water. You can buy hot drinks at numerous tea houses along the way as well of course, but the filter bottle makes a really good stand by and is lighter than a full-on filtration kit.
Alcohol hand cleaner gel seconded. I thoroughly recommend the Trailblazer range of trekking guidebooks, particularly the Everest one by Jamie McGuinness. For background reading - if you're a reader - have a look at Ed Douglas's book, Chomolungma Sings The Blues and pack a very thick novel or two, if you're that way inclined.
As far as clothing goes, think lightweight three-season UK walking kit, but remember it gets very cold in the evening and sitting around during a day, a down jacket isn't strictly essential, but it's a very nice thing to have, ditto a fleece or similar hat and some gloves. The higher you go, the colder it gets. A pair of baselayer leggings double up with trekking pants for colder days and also make good cool weather jim jams.
Good quality sunglasses - remember the atmosphere is thinner at high altitude, so there's lots of UV flying about, ditto sun cream. I like the P20 apply and forget stuff, but supplement with lip block. If you're prone to dry skin, a small tub of something like Netrogena will keep your hands happy
Multi-day friendly baselayers are good news out there. Merino wool works really well in my experience, buty if you're going synthetic, make sure you use stuff that doesn't pong like a dead sheep after one day.
And a properly warm sleeping bag. It's quite cool up at Gorak Shep

It's an aweseome country and a lovely trek, have a great time!