The lack of a complete maintenance history is the reason not to buy any car.
I wouldn't rule out a 600, though, if it had a complete maintenance record and could be had for the right price.
All things considered, if similarly equipped, the only difference between a 600 and a 500 would be the engine. Everything else is basically shared.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".