Best thing to do is ask the locals which antifouling works best where you are. If the present surface is not too bad, sand it down with wet wet and dry and put a couple of coats of whats best on. make sure you get it into all the crevices and areas that are difficult to get at. I normally put an extra coat on areas of high erosion like the leading edge of the fin and skeg.
I recommend any type of Jotun. You can normally pick them up cheap from a fishing gear outlet. I've used them all with good results.
PS my first boat was a shetland 535, good fun!