I assume by the years of the cars, they are either 240Ds or 300Ds, or 300SDs or a combination of those. Those would all be 616 or 617 engines. On those I use a "gear wrench" as noted already - on of those Sears Craftsman 12 mm combination wrenches with a ratcheting feature on the closed end. I have never had a glow plug on one of those beasts break off in the head, and am not sure I have even heard of that happening. The later, aluminum head Diesels, especially the 606 series seem particularly susceptible, but they are mid and late 1990's machines.

The cited tool takes most of the access issues and makes them moot. The last cylinder on a 5 cylinder (next to the firewall) may be difficult even with this wrench - I have never done this on a 5 cylinder so I don't know.

Good luck, lots of ways to get this job done and none are really difficult. Jim

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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)