If your break falls within school holidays, particularly the summer holidays or half terms, book as early as you reasonably can, ideally several months ahead. These are the weeks every family with school-age children is trying to book at the same time, so the best weeks, and often the more sought-after units within those weeks, go first. Waiting until a few weeks before a summer half term usually means choosing from whatever's left rather than what you'd actually prefer.
A static caravan is bigger inside than most people expect from the outside. Most units have a separate living area with a sofa and television, a kitchen with a cooker, fridge and the basics you'd find in a small kitchen at home, and two or three bedrooms depending on the size you've booked. It's closer to a small, self-contained holiday cottage than anything resembling camping.
Choosing between a static caravan break and a hotel stay usually comes down to one question: do you want your holiday to feel like your own space, or do you want everything done for you?
Neither setting rules out the other entirely. Most coastal parks are still within striking distance of countryside walks and market towns inland, and most countryside or
Norfolk Broads holidays parks are usually within a reasonable drive of a beach for the day, so you're rarely choosing one to the total exclusion of the other. It's more a question of which one you want as your daily backdrop and which you're happy to visit as an occasional day trip.
There isn't one single answer to how far ahead you should book, because it depends heavily on when you actually want to go.
If you're travelling with children, pack a beach bag that stays by the door, buckets and spades, a windbreak if you have one, sun cream regardless of the forecast, and a full change of clothes kept in the car rather than the caravan, since sandy, wet children rarely want to walk back for it. Bikes are worth bringing if your park has cycle paths or is close to a coastal trail, and a car boot cool bag makes packed lunches for day trips much easier.