I’m looking to replace my 1983 300d (361,000 and climbing) and am considering an r class, likely an early model, and probably with close to 200k. Am I crazy? Can they be worked on without buying lots of diagnostic equipment? What are the major issues to look out for when test driving? Are there any complicated “must do” maintenance items? (Timing belt/chain, etc) I tried finding a maintenance schedule, but the website listed changing spark plugs which I found odd and disconcerting on a diesel.

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'83 300D turbo diesel (334k daily commuter)
SOLD '80 240D 4 speed manual, #2 piston pitted from crushing a 10mm nut. Running it 'till it burns through.
'78 F-150 300cid 4spd od (with my great unkle's original sales receipt)
SOLD '66 Ford P-350 delivery truck (almost driveable)
'49 Dodge B1D (1 ton Pilothouse era truck) not running but all there, candidate for om617 and 4x4 conversion when money permits.

I've had my R350 for 6 years and about 80,000 miles. I am the third owner and had my indy do a PPI before I commited to purchase. They were familiar with it as the PO had had it serviced with them. The only issues I've had are front shocks, rear air bags and a power steering reservoir leak. One electrical gremlin is my side marker light coming and going.
It's got a ton of room inside, it'll swallow two eight foot kayaks with the seats down. I'm actually thinking of moving to a newer one, 2011 or '12. They aren't that popular so prices are pretty good. I'm not sure about the diesels but the gas engine gets a solid 19 mpg (US gallon) average.

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1985 380SE- 160,000km 'Blistu' - Sold
1988 300CE- Eva,my garage mistress
2007 R350 4matic- The Black Pearl
2011 Ford Ranger-Work truck
1974 25' Trojan Sea Raider- 'Grannyknot'
2002 215 cm Atomic DH skis- Canadian SDH record holder, 175.5km/h- Vars 2010