I've got a bunch of stuff on Sheridans, I downloaded the user manual for them the other day
For a good idea of what can be done with older kit we only have to look to Israel and South Africa in the 70s and 80s. Israel isn't such a good indicator now as they're the 3rd biggest arms exporters, but prior to this they never threw anything away.
Vehicles such as the AMX-30, Leopard 1, M60 are all good vehicles for official upgrade. The European states had a plethora of little armoured cars, light tracked vehicles and so on such as the Panhards, the various Unimog-based armoured cars, and a lot of light tracked vehicles. The
Stormer is a Scorpion upgraded for today's wars, and a good yardstick on how old vehicles could be upgraded.