The DC-DC converter is always running when the car is "on", so no risk of it "kicking in and generating more noise" the converter cycles on and off as needed when the car is "off". While it's hard to know what the total free capacity of the DC-DC converter is, I'd hazard a guess that it's probably over 1000w (though it may vary a bit if using all 5 seat heaters and the rear defrost!)
Honestly, I can't imagine a scenario in amateur radio that I would worry about capacity in this vehicle. And better than any ICE, you have 85kWh of capacity on-tap without starting the engine!
As for capacity to fuse for, etc, that will be based on your gear similarly to any other vehicle or even stationary installation.
This car has more 12V capacity than many standard vehicles, so unless you're running an HF amplifier pushing out a kW or more, I simply wouldn't worry about it.
Well, amplifier is precisely where I'm wondering about it: I'm doing the power-system installation now; but on advice from friends, I'm trying to “amplifier-proof” the setup ahead-of-time. (Avoid re-wiring the entire car, basically, if I ever do decide to install an amp.)
Regarding the draw of smaller items — my understanding is that there's a power-management system in series with the 12V battery, somewhere; and I'm additionally curious how it will react to adding a bunch of parallel loads? (I'm a noob, does that even make sense?)