In 2012 I spent five months in Nepal, five weeks of that in the Kanchenjunga area, going as far northeast to basecamp. I can only say what worked for me. I was there in late February/early March which is late winter, so depending on now far in you go, expect lots of snow.
The farther you get from Taplejung into the conservation area, the more remote you are. Trails are not at all like popular trekking routes, they are rugged and require really good, well-fitting boots. I'd also recommend trying out sock combinations that work. Liners make a difference in preventing blisters. Again, I can only say what worked (really really)well for me--http://www.rei.com/product/847019/lowa-mauria-gtx-flex-hiking-boots-womens
As far as leeches go, I only encountered them in the Langtang area and that was during May's early monsoon season. Never saw any from Kanchenjunga all the way to Khumbu area. Leeches like damp, wet, warm, lower altitudes. This is dry, cold climate.
Kathmandu's air quality is horrible. Once you're away from there though, as others have said, the air is very clear. Yes there are yak dung fires going, but it's manageable and you get used to it (unless you are sensitive to any smoke. The sky is also clear beyond what anyone could ever imagine!
Your daughter's trip organizers will surely give you more information about what to bring, whether you'll have porters carrying loads (I would imagine so) and how water will be provided. Bottled water is fairly cheap in main trekking areas, but not in Kanchenjunga area going northeast from Taplejung (foot travel starts there). When I was there Feb/March we never saw another non-Nepali person for five weeks. We took a bus to Taplejung from Baratnagar because there were no flights since it was outside of tourist/trekking season.
Take panti-liners--extends the life of underwear for many days! It's amazing how good you can get, though, at taking a "bath" with just a pan of water, with most of your clothes still on because it's so cold.
Here's some pictures I took while in Kanchenjunga region which might help you (some are a bit west of there):
So envious, would love to go back! Have fun!
Info on gear, conditioning, and preparation for hiking/climbing.
The measure of a person's real character is what he would do if he knew he would never be found out.
Life as a Wild Child
Life as a Wild Child
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