I would agree that its convenient to return to Thailand (Bangkok) especially to organise further travel. When i travelled i neede to go from Koh Chang to Hua Hin and after longer search came to conclusion that the price will be 2 times less if to go through Bangkok. They have great fares for buses and air fares from Bangkok to every part of Thailand. Concerning meals- you can eat for 5 dollars easily. Sometimes it can be more, because there are so many things you would want to taste. Enjoy the trip. Ayuthaya is great, you would love there;-) If you need accomodation in Bangkok I used Royal Hotel, it has great location- everything is 100-300 meters away and you wont have to spend lots of time in traffic. There are not a lot of hotels in historical area of Bangkok, only budget rooms on Khao San road.
Yes, I believe you can take the train from Chengdu to Lhasa. Please make your reservations in advance though.
You've got some good responses. The most expensive part of your trip will probably be in Japan and Fiji. Japan is just expensive period! I would check the lonely planet also but when I was there last, even lunch cost me around $10 so.
Cambodia has many Tuk Tuks (rental taxis) that you can take in terms of transportaiton or you can hire drivers.
Most of the S. Asian countries do have 30 day visas I believe but you may want to double check that -- and check in terms of re-entering the country as I know you asked that.
There are good around the world fares or are you planning on booking all of this separately? If so, Asia.com has some of the best fares I found. Some of the other sites I would suggest is to actually book with an asian airline like Thai air as they will offer better routes and better fares.