more reach is better. you will need AT LEAST 400mm for surfing. Ideal is 500mm, best is 600mm. But since budget is your limited, 400mm f/5.6 is the best choice. 100-400 is also good due to the zoom ability but if you don't have a bright sunny day then you're done.
here is my surfing gallery:
check unders sport. 90% of picture there taken with 600IS lens.
If I only shot surfing, then the 400 f/5.6 would be the best choice for a modest budget. however, the flexabilty of the 100-400 allows me much more scope with other subjects
My surfing gallerywith the 100-400