It depends whether you get a W163 or W164 500.
The 3 valve 5 litre motor is bulletproof so long as it is serviced and so is the 5.5 litre in the W164.
Having driven behind both motors for a very long time I can say that in normal driving there is not much to notice that is different unless you really give the 5.5 it's head then the extra HP is noticeable.
The lifetime fuel consumption was 14.5 litres/100km in my ML 500 and about 12.2 L/100 km in my recently sold E500 - weight would probably make more difference to the figure than the engine.
On a trip the ML 500 would drop to about 11 L/100km with the occasional dip to 10 l/100km while motor way cruising whereas the E500 would do 10 L/100km and dip to 9 L/100km - I once saw 8.8 l/100km on a 2 hour motorway cruise at 110km/hr.
I cannot remember a single repair required on my ML 500 and the only repair I had on my E500 was the mousewheel for the comand breaking it's shaft a few months ago - but even that is a $1,000 + experience.
The Diesel had a problem with the cold start Glow plug relay which was never fixed under warranty - although two sets of glow plugs were replaced at M-B's expense. It was finally fixed at my expense for $1700 which is the cost of a new relay and replacement of 6 Glow plugs. I also had the electric fan fail after warranty had expired and that was a $2400 experience.
Other than that you are looking at the total repairs as opposed to maintenance costs of three cars totalling nearly 300,000 km.
The 163 ML270 CDEI was quite reliable but needed new rear brake pads every 18,000 km which was crazy considering it only needed it's oil changed every 28,000 km. I kept a set of genuine pads at home and changed them myself every time the light came on.
Cheers
Bill