It doesn't appear that your limited compressed air consumption really requires an oil/water separator accessory, but they can contribute to longer life for air hoses and tools. I haven't used any of the newer inline disposable cartridge-type filters.
My primary use of oil/water separators was for spraying automotive refinishing products. These types of separators cause suspended particles and droplets of oil/moisture to be separated and trapped in the bowl, which is drained regularly. Clean air is important for painting when using lubricated piston compressors, to prevent oil/water contamination of the paint products. A final oil/water separator and large cartridge filter are typically located in the paint booth.
The oil/water separators won't cause an airline pressure drop if they are selected to flow a higher volume of air than the tools demand.. generally, separators are rated for flow, by the better quality manufacturers. My separators had 1/2" pipe inlets/outlets, and were rated for much more flow than my air tools or paint guns would consume.
In properly plumbed commercial/industrial air supply applications, all of the pipe is sloped back to the air compressor receiver/tank. As the compressed air cools as it's being distributed, much of the suspended moisture condenses and drops drops out of the air flow, and is supposed to drain back into the receiver, where it's easily drained.
If the air is cooled by the time it reaches the (machine or) end user, much of the moisture has been removed, and a separator is mostly just added insurance against damage from rust scale, water or eventual accumulated oil in the pipes.
In many home shop applications which aren't plumbed with pipe, the user is close to the heated compressed air source.. then as the air passes thru the air hose laying on a cool concrete floor, the moisture begins dropping out of the air flow. Cooling the air will cause moisture to accumulate, and contribute to failure of air hoses from internal exposure to oil and water.
Having overhead pipe giong to several shop locations is very beneficial in terms of convenience and tool performance, and the pipe also increases the air tank's volume. The down pipes at workstations/outlets should be tapped off of the top side of the horizontal supply pipe.. and include a shutoff valve, and a length of drip leg (with a petcock) extending below the tap for a workstation oil/water separator and pressure regulator.. a lubricator and a high-side quick connector are optional. If the hose quick connector is elbowed to point downward, this allows a hose to just drop down to the floor, preventing excessive strain on the ends of hoses.
A handy location for an oil/water separator is in a protective cage on a work cart.. so an air hose can supply the separator nearest the point of use, then the user generally only needs a short section of hose which can be stored on the cart.