I really, really rate Goldstone primary. It has big playing fields, an allotment, great music/drama/art, fabulous teachers, and although it's quite big, it feels small. By week 2 of reception the head teacher knew every child's name, and he goes through the school asking children what they did at the weekend, how's your brother, did you get the puppy, etc. It's in Hangleton, north Hove. Also has a nursery attached which is good as well.
Activities - see if you can pick up a copy of (free) ABC magazine, it's a parents' listing magazine. Try (free) Baby Boogie sessions at B&H libraries, loads of music/movement/craft/yoga classes. There are several really lovely parks and 3 good playgrounds right next to the beach, lots of farms to visit in the area, etc.
To pick an area, what worked best for us was to spend a morning driving around and picking up the 'vibes' from each area; decide whether your priority is to be near the stations, near the sea, in the middle of the action or in more peaceful area. Prices do vary quite a lot if you are buying, and the one very big factor is garden sizes - the area I am in has big houses but very small gardens.