You could erect a building the size of a warehouse
Your smallholding will need a building and you will certainly need a barn and perhaps a polytunnel if you plan to grow anything. You should build the largest barn that you can afford as this will be the hub of the smallholding.
If your aim is to grow anything on any scale then a polytunnel is a must. These structures normally need planning permission, however as the book demonstrates there is a way around this. You could build more than one, in fact, you could have a barn and a polytunnel. Without planning permission you may build a new barn or polytunnel every 2 years as your business grows.
The barn will be a steel frame construction clad in either wood slats or in plastic coated steel sheets. These buildings can be bought in kit form, or fully built, or at any stage in between, the choice is yours. The amount of input you have is up to you. I would suggest that the best option would be to get the frame up and roof on and clad by contractors. Leaving the bottom 2 metres from the ground, this 2 metres will be made up by concrete blocks laid by you. The barn is most important as all the services will come into the barn and be redistributed as required to the mobile home and to the caravan park.