You will have to sit him down when you tell him this. In the US a running Sherman tank starts around $200,000. If it is a particularly desireable model and a very well done restoration, try $500,000. Universal Carriers are attractive and the price is rising. They are also fussy little creatures that need love, care and massage to perform at expectations.
The least expensive alternative is to go for a former Warsaw Pact tracked vehicle that is not a tank. Vehicles like a BMP or MTLB ( sovietarmor.com/for_sale.html ) are simpler than British vehicles, even though the instructions and parts are all in Cyrllic. American vehicles have too many Arms Control restrictions. German vehicles are either postwar and scarce, or wartime and unbelievably expensive.
Another source of information is milweb.net/classifieds.php.
Good luck, and let us know what he decides !
