Yeah I've always let the glowplug light go off prior to cranking, unless the engine is already warmed up. I think it runs smoother if the glowplugs are hot, but I could be wrong. The few times I "forgot I had a diesel" and cranked mine prior to warmed up glow plugs I had to crank 2 or 3 seconds prior to the engine firing. I think this was in order to build up heat/compression in the engine.

If the person is reputable and has nothing to hide they should let you take the car (or take it themselves and you pay for it) to a Bosch service center, MBZ dealer, or independent MB mechanic for a pre-purchase inspection. It costs around $50 around here. Good luck!

__________________
Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic