A few things,
First get your engine up to snuff.
here is a good link
Mercedes Diesel Maintenance tips
My 85 300SD has basically the same engine. To get good mileage on an OM617 the engine needs to breath. If they don't breath well you make soot from your fuel instead of power.
1) The first thing you should do is check the simple things. check your air filter and possibly replace your fuel filter.
2) Adjust your valves or have someone do it for you. Its best done every 15,000 miles
Mercedes Diesel Valve Adjustment Procedure
Note dieselgiant also has a video showing the difference this can make.
3)The next item is to check your max boost pressure to make sure its in spec (11-12.5 psi). If not you need to adjust your waste gate which is kind of a pain but well worth it.
4) 85's have an Air Recirculation Valve (ARV) on the turbo. Plugging the ARV vacuum line on the compressor housing of the turbo will get you a little better mileage in town as well as a slight boost in power at lower rpms ( higher rpms will remain uneffected).
5) With the 85 300D there is no direct mechanical linkage to the fan. It uses a viscous fluid coupling that only firms up when it gets hot. If it is working right most of the time it free wheels. Changing to an electrical fan would be less efficient. To keep the fan free wheeling as much as possible do a citrus flush.
Now back to aero
I would also say forget the airdam. I never met a w123 4 door owner that benefited from one as far as mileage (handling is a bit different story though).
If you want to do wheel strakes I would suggest making them out of broom bristles. You can trim them to fit. They won't break if you climb over a curb inadvertently. They are also cheap.