The thing with the V-8 chains is they are long - six feet long! That length and route they travel causes the chain to make many turns as the chain goes around the crank up to the head, around the cam(s) back down to the center and around another gear then up to the other cam(s) and back down to the crank. That length and route when coupled with infrequent oil changes really wears the pins.
On the other hand, the smaller 6's the chain is short and only make two turns, at the crank and the cam(s). They nearly last forever.
The plastic rails get old, discolor and become brittle. When brittle they break easily. Every 420 I have seen with a broken chain also had a broken rail in the left head and when the chain broke it wiped out the left head valve train (cyls 5,6,7,8). Usually what caused the chain to break was a broken rail lodging a chunk in the sprocket which broke the chain.
The only way to tell the condition is by pulling the valve covers if the matter hasn't been addressed and is in the records.
If you are mechanically inclined you could do the chain yourself, as well as guides and tensioner and drip tubes for about $250 in parts. If not, then try to negotiate this preventative job as part of the purchase - they do the chain etc, you'll buy it.
If you don't change the chain, guides etc, what could be a $1,000 job (if you don't DIY) will quadruple as you replace chain, guides, tensioner valves, cam(s), towers, maybe pistons and whatever else gets mangled when everything gets out of sync.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.