The old Tamiya tanks from the 1980's cost a small fortune when new and sold in relatively low numbers - decent examples are hard to find today and even the poor examples command good prices as people seek them out for parts to restore another model. I recall buying a 58028 Toyota Hi-Lux around 1982 which was about £130 at the time and the tank kits were probably double that, maybe more.
Maybe think about Meccano ? Plenty of it about.
Building a tank from Meccano is probably a good project for him to tackle - plenty of engineering potential and no problem sourcing the rare spare parts as you would have with a vintage Tamiya model.