I can't help you much with where to buy or what model, but might have alittle insight after a some travels around China the last couple years.
250 is about as big as they get, and at that you would be looked at as one with a big bike. Considering the cities in which they ride, I'm sure the 250 will feel like a big bike too. Small and maneuverable are keys to getting around most of the time. Speeds rarely get over 45 mph, even out in the country. Part of that is the state of the roads.
A quick search on "Tainjin motorcycle" ended with this old press release and several other references. So I'm sure there are some good options in the area.
http://world.honda.com/news/2002/2020614.html
Good luck and have fun on the trip. Lots of interesting things to see and eat there. I enjoyed my time. Oh, and
