Its app 700km or less from Bangkok as you need to get to Don Sak (east of Suratthani) to board the ferry.
I think there is a boat from Chumpoon too, but that one dont take vehicles.
Great write up abg acid..... I'm a newbie here and planing to ride to Ko Samui and go to Koh Phanggan as well but not sure with bike or not...need to consider now after reading your post...