Hi Ian,
to the forum.
There is heaps of accomodation on Samui and at that time of year it is not necessary to book in advance. But, as it is a slow time, you may find some great deals on the main booking sites who will be trying to fill rooms.
In you budget, I would recommend this place....
Samui Beach Apartments, Koh Samui hotels | Sawadee.com, Thailand
Great central location and access to all the facilities of Seascape resort, next door. Good sized rooms, clean and comfortable.