Hi! I think that the rules (and the developer rulings) in this case are quite clear. You should do as much as you can, even if that means only spending one or two resources. In fact, the rules (version 1.6) for the Choose keyword say:
When an encounter card requires a player to choose an option, they must choose an option they can resolve. If they cannot fully resolve any option, they must choose the option they can most fully resolve.
This is specific for encounter cards. It is not the same as spending resources for an action or to pay for a card (in which case I agree that either you can or cannot).
Thanks!