We bought a static caravan years ago but it was already sited. They were very strict about the age of caravans, after about 8 years they had to be removed so I'm not sure if the sites would allow you to bring your own onto the site. It also became very expensive, the annual charges even then were phenominal and if you sold your caravan they took a percentage, something like 15-20%, of the selling price.
When we lived in Germany and had a touring caravan we would leave it on one site for most of the season and that worked out cheaper.