If you are talking about just 200psi, then it would be a lot safer. However the quantity (volume) of air needed will be huge because not get much work will come out out of just 200psi. Why don't you study steam engines. An air engine is, to put it crudely, somewhere between an IC engine and a steam engine.
An expansion engine is basically a two stroke engine with valves. If you are thinking of modifying a normal 4 stroke engine, the main modification will be to the camshaft. The inlet valve will open at the beginning of the intake stroke just like in a normal engine. The next stroke instead of being a compression stoke will be the exhaust stroke, i.e. the exhaust valve will open to exhaust the spent air and the cycle will begin again. Converting a Diesel engine is more sited to an expansion engine due to its higher compression ratio. Higher the compression ratio the more work you can get out of the expansion engine.
Yes the pressure differential is what causes the air to go cold. Temperature, pressure and volume are related according to Charles and Boyles law pV = nRT. The act of increasing the volume of air causes the pressure and temperature to drop.
The engines I worked on were in a plant that manufactured liquid Oxygen and Nitrogen by cooling th air to -196 degrees C. Oxygen which liquefies at -186 degrees C is separated out first and then upon further dropping of pressure, Nitrogen is liquefied. Expanding air from 2000 psi to 80 psi gets to near liquid air temperatures. As a mater of interest, expansion engines need a constant load to keep the RPM in check. The engines I am talking about had electrical generators to load the engines. Electricity generated was fed back into the grid.