where i have my van its a 12mth licence but you can only stay 28 days at a time then have to have a break, but i can use it all year round rather than march to october. tbh it sounds more that you are paying for the land that happens to have a van on it, but as stated land will have a yearly council tax on it or an agricultural tax on it (farming/nursery/woodland) and you could find that once purchased the council make you remove said van.
Keeping a few chickens will not give you right of occupancy as a lot of folk seem to think