Cheers Carlton and welcome to the site.

Any place where fuel can leak out is place where air can leak in when you shut down. This will make it necessary to purge the system before it will start, which sounds a bit like your problem. Repeated use of the starter for periods of a minute or more will wear the starter out.

A starter is designed for "intermittent duty" so it draws current to accelerate the engine to its starting speed quickly, then maintains that speed until the engine starts. This should typically involve a few seconds, and the starter motor never gets a chance to get really hot. Using this means to rate electric motors that see intermittent duty only saves cost and weight. However, running these machines at their itermittent duty rating for longer periods of time, repeatedly, ends up seriously overloading them.

Running for a minute at a time, time after time with little cooling off time between operating periods will bring the starter motor to a temperature approaching the temperature it would reach at constant duty at that load. Motors are rated by the temperature the winding reaches under load that is considered safe for the design life. In the starter motor application the load is not acceptable for the motor in constant duty and the safe temperature is rapidly exceeded. When that hapens the insulation is degraded and that causes losses that limit the available torque (which means it limits the starting speed of the motor as it tries to crank the car engine over). This, in turn, limits the likelihood of a successful starting attempt, leading to more use of the starter and so on until the starter is no longer useful.

So, you may have an air leak into the system when it is shut down, that is also causing you to wear out your starter motor. In any case, I would fix the apparent air/fuel leak first and then see how the machine performs. If all else checks out (glow plugs, clean fuel filters, clean air filter, correct viscosity oil, etc.) I would suspect the starter motor. Good luck and I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)