For a start I don't recommend Phi Phi, unless you like overbuilt, gap year piss head infested holes.
I've never been to Khao Lak, but from what I read, it's seems nice, though quiet.
+-8 hours between the ferries
???????? Car ferries to Samui run frequently during the day.
although it seems very overpriced for this time of the year
Most of Samui is overpriced at any time of year.
Phangan has far cheaper options, but you get what you pay for.

