Thailand forum

Arrival by flight at Phuket yes you'll need to stay a night somewhere, but taxis/tuktuks on Phuket are extortionately expensive and the airport is at the northern part of the island away from the busy beaches of Patong/Karon etc.

From Phuket town to Surat Thani is about 240km so I'd expect a bus to do that in about 4-5 hours depending on route and number of stops. It will be important to know where that bus terminates - Suart Thani town, the transport terminal at Phunphin (near Surat rail station and airport) or all the way out to Don Sak pier where ferries leave for the islands. Makes quite a big difference in how many and what type of connections you'll have to make. Last ferry is usually 6pm but Don Sak is a good 80 km past the other stops.

Sure would be a lot cheaper/easier if you could fly into Surat Thani instead of Phuket.

You could get a combined bus/ferry ticket through an agent in Phuket or go to the bus terminal in Phuket and get a bus to Surat Thani .
Then once in Surat go to an agent where you will then get a combined bus/ferry ticket to Don Sak pier/Samui.
The direct bus which goes to Don Sak pier is at 10am. If you take a bus from the bus terminal there is a bus to Surat Thani almost every hour. But you will need to get the earliest possible if doing it this way. The pier is one hour from Surat Thani town.