Speaking to a good trusted indy is the best way to get an understanding of the likely costs.
My local indy (Liam Huxley in Fleet Hampshire) was very helpful before I bought my CLK. I had a chat with him to understand what I should be looking for specifically. We discussed issues with the 350 engine, and ascertained that the car I was looking at was outside the danger zone. I asked him roughly what the cost to run and he estimated £500 - £1000 on average but stressed that unexpected big bills can arise. I've had the car nearly 4 years and that estimate has proven to be bang on the money.
As soon as I bought the car, I took it to Liam and he spent half an hour crawling over it and listing what needed to be done to get it up to scratch. We went over the car together and he showed me everything that was needed - nothing major, but stuff like discs and pads all round, tyres, faulty thermostat, intermittent SRS fault, steering geometry, bushes etc all needed attending to. Seem to recall a bent track rod arm too.
We did that work over the first 6 months I had the car and it cost me £1,400 in total using genuine MB part. By that point my wife was going mental at the amount it was costing.
But once the car was sorted, it settled down. The next 18 months cost around £500. The third year of ownership required the major B service, and it needed some suspension work due to pot hole damage. That year I spent around £900 on it. This year, it has just been serviced at the bill was £300 all in with no particular issues to note.
So, on average, the £500 to £1,000 estimate is about right.
The one thing which you can do to cover yourself is find a well looked after car. Mine is pretty much as it left the show room and I think that's why I have had no nasty surprises. Condition is everything as is evidence of proper maintenance.
All of the above is based on a 3.5 litre CLK, so an ML will have different costs. But not dramatically different I wouldn't have thought as the gear box and basics are not so different.
Good luck with your search.