Is the system only going to be powered while the car is running? Is this just for an experiment or something that you want to optimize? For example, are you wanting to go quick and dirty (literally, in the sense of power) with an inverter that converts 12V DC of the car's charging system to 230 VAC as a pseudo sinewave, or do you want to power bypass the internal server power supply and power it directly with filtered 12V and DC to DC converter 5V and 3.3V (any other power forms your server needs?)?
What "temperature rise" are you wanting to prevent? Temp rise in the server due to interior temperatures while the car is parked, or temperature rise in the van due to the running server?
As for determining if the van can produce the required power, are you familiar with Ohm's Law? Your answer on if you plan to use an inverter or DC power will help determine if the car can provide the power, but you'll also need to understand how much power the van typically uses versus the rating of the alternator.