There are a number of compressor options out there that actually run on DC (The box type you said you don't really want, but they have their place), and I have an
Orcair Compressor that comes with DC cables for your car. The DC power supply inside it is rated for 40A at 12VDC, so it is 480W. You will have to stick to lower volume with a compressor like this, which is fine to direct fill your gun in the field.
As has been mentioned, the inrush current when getting the motor started moving is what you have to be careful of. This current is generally about 3x the load (for a split second) as it is when running. If you put an oscilloscope on it with a fast enough trigger, you will see that the current gets insanely high for a split second because the coil of that motor is a dead short for a second while the magnetic field is building up. Another project I am working on that has a compressor (That I have the specifications on my desk right now) runs on 85A continuous, but it has 285A inrush current. All this said, these values are instantaneous, so the mechanical mass of your circuit breaker can't trip fast enough that the circuit will equalize before it can trip. You are essentially banking off of the capacitance of the power lines to start your motor. The same can be said about the DC systems. The battery on a car is a huge capacitor that can handle the monster inrush required by the starter motor, so the same would be true for a compressor.
As an example, the Orcair compressor is rated at 250W, which is 20.8A at 12VDC. According to the ratio of inrush to running of my current project compressor, the inrush is 3.35X the running current. My compressor is pulling 70A from a supply that is only rated for 40A!! The same can be said for Biohazzardman's YH compressor pulling 60A from his wall...but he only has a 20A breaker!!! This is the reason why you can NEVER use a fast-blow fuse on a motor (inductive) circuit. You really are demanding that much power when you are running the numbers, but most circuits are designed to provide that much power before tripping to account for this commonly required inrush current.
As a general rule, if you want to run a DC source off of a car battery, and the current required is too much for the battery itself, take a suggestion from the car audio guys and put a bank of big electrolytic capacitors right next to the compressor/inverter with big (heavy gauge) wire going to the caps. They will provide the electrons necessary to get the motor turning and the battery won't even know it happened.
To summarize, while you will have to consider the inrush current, it is instantaneous, so the running current is more important if the conditions are right. You can add capacitance to increase your likelihood of success.
I hope that this helps. I don't think you have to spend $3K on this.
Jonathan
