1) There are many jobs online, ESL seems to be a popular one. There are a few fake schools online so be sure to get further information (fixed telephone number, contacts of current teachers working there, photos of accommodation). Some schools prefer couples I am not sure on the issue of couples, you can always google "couples teacher China" or something similar.
2) Airfare and accommodation are usually included as part of the contract, but be careful and assess the entire package provided i.e. total pay+salary+travel allowance+housing+bills for housing+food. Each of these may or may not be provided. So some schools could give a big salary and provide no accommodation. Do the math!
3) As far as I know in China schools recruit for the beginning of September and March although this may change slightly according to the school level (Primary, middle, college). This means you have to apply about 2/3 months before this date.
4) Some schools want TEFL qualification, some don't. Every recruiter will have his own personal requirements.