I think you will end up doing less miles than you think you will. Although driving a CVRT on the roads can be enjoyable, when you have got to get home, it's November, raining, dark and you still have 30 miles to go it can be hard work. You won't be using it to go and pick up some milk from the shop, or doing a trip to the beach for fish and chips like you might a classic car.
If you are going to visit Andrew at the Alvis Fighting Vehicle Society you will be visiting one of the largest stocks of spares. How do you define sensible cost? Track cost the MoD something like £18,000 a set, prices on the civilian market can vary from a few hundred £ for some take off track to thousands for new.
You can't buy track at ATS, when you need it you have to buy what is available. If you don't need it, and it becomes available you need to have the money and space to store it. This applies to most parts for these vehicles. You also have to remember they are still in service with several armed forces around the world so any new spares which become available tend to get hoovered up by dealers and exported.
How soon are you looking to buy? It would be worth visiting some of the larger shows such as War & Peace as you'll be able to see a good selection of vehicles in the same place and talk to the owners.
Chris