My criteria: Enough armor to ignore crossbows, self-propelled, amphibious, packs at least a .50-cal, fairly simple and reliable, air-conditioning, doesn't need a crew to operate, can fit through most city streets. A DUKW with a Ma Deuce on the pintle mount would fulfill most of these requirements except for the air-conditioning & the need for a gunner; say add an armored hydraulic turret, can elevate to near-vertical for air-defense with dual Ma Deuce & remote-control joystick/trigger -- wouldn't want to add so much weight it'd compromise amphibious operation -- mount the turret in the truck bed and fit big ammo boxes to feed the guns. Perhaps a Gatling instead, lose a little redundancy but less visibly military, keeps the WW2 level technology.