A business visa for 70 days costs about $50 USD. To get this you can be in the country but you will need a letter from your employer so it’s best to get a job organized first.
To get into the country as a tourist you will need to get your visa beforehand (although there are plans to allow for visas on arrival). It is wise not to stay beyond the visa expiry date as this can cause problems.
There is no work permit or teacher’s license required. Almost all jobs will ask for a TEFL Certificate and some, though by no means all, will also ask for a degree.