I read that 100K-150K is the safe life of a V8 chain. I guess it depends on whether you replace just the chain or everything that can be replaced.
I don't know how much a full chain job costs. I'd jump on $700 in this area if it includes guides and a tensioner. That's too much to just roll in a new chain which could be all you need. I'd check the condition of the water pump and thermostat while the cooling system is half apart for a major chain job.
I'm not sure what valve seals costs but $400 sounds fair.
I take it you're not a DIYer. You could do these jobs yourself for the cost of parts and a couple of common tools.
I gave up my 450SEL a couple of years ago so I'm out of the M116/7 loop these days.