While originally written back in 1999 (I was a co-author) and it was for the 560 coupes, plenty of it is very pertinent even today, for the 420 SEL:

http://www.mbcoupes.com/buyersguide/buyersguide.htm

Worth a look.^^^^^

Along with the timing chain,

the timing chain tensioner and guide rails should be replaced at 129k, if there are no books and records to show it's been done, IMO.

The website sponsor, FASTLANE, would be worth checking with for the parts.
(Also, Rusty Cullins in Atlanta and Randy at EPS online are good sources that I have used in the past)

No way to check really, unless you remove the left hand valve cover and peer inside. The guides are white, when new, but over the years they develop hairline cracks, which finally get worse. If they look root beer brown in color, then they are really old. If the car rattles for a bit when you start it up, then quiets down, it might possibly mean the chain is loose and about to let go. If it does, at speed, it will screw up your valve train really bad. The price of a new engine could be $5,000 to $7,000, maybe even more for a crate long block, and that'd be more than the car was worth.

If it's not rusty or a refugee from a freeway pileup, and your mechanic checks it out and it's good, it sounds like a great price for the car.

__________________
1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)