I have never really done a trip requiring so much flying. Usually, I fly into a country and make my way overland to other countries on my itinerary.
This site might be a good lace to start (though I haven;t used it):http://tripplanner.airtreks.com/HtmlUi?ref=BOOTSNALL&dst=RTWT
Regarding the ticket, I would probably shop around, maybe visit a travel agency, see what they can offer, then compare to online services, probably look into a round the world type ticket.
Of the countries you list, I have only been to Nepal, China, and Thailand, and only Nepal recently. I do heartily recommend China. When I was there in 1996, it was possible to spend a night camping on the Great Wall at the Simatai location, and Beijing is a fantastic city. If you are in China for a limited time, I think that it is a perfect destination. Xi'an is interesting, and the Terracotta warriors are definitely worth seeing, though the air pollution was pretty bad when I was there (it was, a major producer of coal). I have read that coal-production was halted in 2002, so it has likely improved.
I was in Nepal from September 2010 to March 2011. It is a wonderful country (my favorite) and incredibly cheap. I recommend the Langtang trek, which is very near Kathmandu. I spent about a month on the trek, including the newly opened Tamang Heritage Trail, the Gosainkunda trek, and finally the Helambu trek, which takes you back to Kathmandu. The trek begins with a rather hellish bus ride from Kathmandu to Syabrubesi, but it is worth it. A month is excessive (I went slowly and savored it), and the trek can be tailored to fit tighter schedules... the Tamang trek can be eliminated, for example, and you could simply do a Langtang circuit... if you can stomach the bus ride twice. You do not really need a guide, and I have never used one for any trek, but it might be interesting to hire one.
I have also done the Annapurna circuit, which I recommend, but have not yet had a chance to tackle the Everest trek. The out of the way Nar-Phu trek, along the Annapurna circuit, is definitely worth doing, for a less touristy experience. I did not encounter any other trekkers, and crossing the Kang La pass was spectacular. The Rara lake trek, in far western Nepal, is also amazing, but a bit out of the way for a three month trip thorough Asia.
I wish I could be of more assistance. Good luck and have a wonderful trip!