Car rental
I use Budget in Phuket for 2 reasons, on longer rentals, I think 5 days, they offer no drop off fee anywhere in the country. I have dropped at off from Phuket in Hua Hin and Samui. From Krabi (via Khao Lak) at Phuket airport. They run frequent specials for a small Toyota or Honda at about $35 per day. The other reason is that I find the staff at the Phuket Airport Avis location to be obnoxious and very un-Thai so have written them off, also their drop-off fees are outrageous. Both are across the street from the terminal. IMHO a 4 wheel drive is unnecessary for most normal travel. Thai infrastructure is quite good.
In respect to driving my advice is to be aggressive and defensive. Never have noticed that center lines have any meaning. The white line on the side does differentiate cycle traffic from bigger stuff but do not be surprised by traffic moving toward you when you think it shouldn't. It is not unusual on 2 lane roads for driver's to pass in both directions at the same time. Move over, and remember the bigger you are the higher your precedence. Also believe that if it is your time to go, it may be. Just relax and go with the flow and never challenge a Truck unless you are in a bigger one.