I just called my mechanic, he gave me a price of about $350, including parts, to change the chain, tensioner, and upper rails. The lower rails would be much more difficult to get to, and he's never seen them break off. He said with 46k, I probably shouldn't bother.
Another tip I got was that after the oil is changed, the lack of lubrication causes the tensioner loosen and release the chain onto the rail somewhat, and when you first start up the now-dry engine after refilling the oil, the chain slaps against the rail, which can cause it to break off - potentially causing a problem. He suggested disconnecting the ignition wire after oil changes, and cranking the engine first without a spark to build up oil pressure, then reconnect and start.
2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino