Asset sheets linked to faction sheet as inventory. This will be standard and requires no custom approach.
Asset sheets linked to location sheet as a simple document link (much like when an item is dragged to a journal entry). This will require overriding the default drag-and-drop behavior for this particular actor type.
Base of Influence should have boolean value marking them as such
Damage to Base of Influence assets should automatically deal damage to owning faction
Standard attack/counterattack should be handled automatically by selecting the defending asset. Preferably use a Faction -> Location -> Asset dropdown approach to make finding a particular asset less confusing. May need asset dropdown to display current asset HP, in the case of identical assets in a given location.
Three related sheets:
Features and general approach: