Of course the airport will not make it easier to leave paradise. In Samui the weather is very unpredictable. We can't really say when is the best period to come. Normally the rainy season start from the end of October until middle of December. The coldest time is in January, sometimes reach 18 degree Celsius in the night.
Short thunderstorms are very common all year round.
We can say that is possible to come in Samui all year round. In Samui there are several kind of transportation, the cheaper way is to use the public taxis which run around the island from 5am until 6.30pm. Other way of transportation is by using private taxis which run around the island 24 hours a day. In the night only private taxis are available. If you want to be more independent is possible to rent bikes or jeeps.