New owner gas smell

Congratulations on your Prius purchase.

Empty engine coolant reservoir: Use Toyota Super Long Life Coolant to replenish. Inspect the engine coolant pump bearing for whitish or pinkish coolant stains on the pump body or deposited on the air conditioner compressor housing beneath the coolant pump. If you see any signs of coolant leaking, replace the engine coolant pump.

Gasoline fumes: You should never smell this. There are no associated recalls. As you suggested, the fuel injector O-rings may be at fault although this has not really been reported as an issue. Make sure the air cleaner plastic housing has no cracks or faults and check the tightness of the clamp that attaches the bottom of the housing to the throttle body. The lid of the plastic housing has a charcoal filter which is supposed to catch HC fumes coming up from the throttle body.

While you are looking around, look at the condition of the air filter (the bottom is going to be the dirty side), and inspect the throttle body plate and throttle body interior. If you see blackish stains in the throttle body interior, clean that out. Remove the air cleaner housing from the throttle body for access. Use a clean cloth moistened with throttle body cleaner and scrub out the dirt in the throttle body.

Don't drown the throttle body in cleaner fluid. This causes two problems: 1) dirty fluid will run along the throttle plate shaft and get into the throttle motor. 2) the fluid will have to be consumed by the engine which causes unnecessary pollution.

Finally, look at the mass air flow sensor mounted on the air cleaner housing. If necessary, remove the sensor and clean it using MAF sensor spray. Don't use throttle body cleaner as that contains lubricant which will attract dust and dirt.

The intake manifold below the throttle body will probably show a pooling of hydrocarbon deposits. Don't worry about that, it comes from the engine Atkinson cycle which allows some unburnt air/fuel mixture from the combustion chambers to come up into the intake manifold. (The intake valves remain partially open while the compression stroke is rising, hence some air/fuel mixture can enter the intake manifold.)