To stop the engine,just push the lever on the IP where the cable is attached forward to full stop(that`s waht the cable actually does each time one pushes the knob). Do not hurry to change the Push/Pull starting mechanism-inspect the area where the cable connects to the IP body-have a look at this thread:

http://www.peachparts.com/shopforum/showthread.php?t=240306&page=4

As Diesel911 said,clean the engineand inspect-the front and rear crank seal area,the oil filter area,the front face where the oil pressure relief valve is,the cap of the idler sprocket bearing bushing...gaskets...

Upgrade to the pencil-type plugs(takes just the 4 plugs+home made wires -you do not need a relais:
http://www.dieselgiant.com/mercedesdieselloopglowplugrepair.htm
...don`t forget to remove the grounding cable from plug #1 to the cyl.head...
If you have the ballast resistor(thick wire coil)monted on the firewall,by-pass it...salt shaker too..
Voltage to your type of glow plugs:
http://www.hehlhans.de/bilder/autos/g240/reihe.jpg


You probably have air in the clutch system-bleeding is a bit tricky-lots of info on this forum-like this thread:
http://www.peachparts.com/shopforum/showthread.php?t=215891
...or the rubbers in the cyl.need replacing...

Change all the fluids and filters with what the manufacturer recommends..change the oil in the IP too.
I`d also overhaul the brake calipers and change all the brake hoses...pretty straight-forward...unless you have a stuck piston..
Similar to Porsche calipers:
http://www.pelicanparts.com/techarticles/911_caliper_rebuild/911_caliper_rebuild.htm

Here a beautifully restored /8:
http://www.zeitlos-oldtimer.de/angebote_druck_detail.php?id=28