Generally speaking you should be ok, but the
weather has been strange world wide. The south is
not my strong point but anything that you would
need you can purchase cheaply there such as a
rain poncho or umbrella. It is a hot climate so
you don't need too much.
Check this out for more info http://phuket.com/island/climate.htm
I needed a place right at the airport in BKK and didn't want to spend a fortune and got a room over the net. It was adequate for a night (which is all I needed) and cheap. I had no problems. I would suggest that you check to see if the site has an authorization # from the TAT (tourist authority of Thailand)and use a major credit card. Usually I just wing it when I get there.