Bigfatsi,
Your vehicle is just fine the way it is, so long as it is road legal. The only thing I would suggest you do is get some good tyres on it. We have several bog standard discoverys and defenders and they are managing fine on the training course. A good set of AT tyres is always a bonus though.
With regards insurance, you only need tell them that you are volunteering to help a search and Rescue group. Thats alll you need to do.
Go look at the website and print off a membership form, fill it in, send it away. You can always email me or the any of the committee from the website.
Hope to see you soon.
Chris