I think ISO-FIX mounts were standard for the last few years of production of the original ML. We have them in our ML, but not the C32.
They only (used to) cost £30 when adding them specially to a vehicle spec, which makes me think that the cost of putting them in every car would probably negligable (spelling??). I think they ought to be mandatory too - given how important safety is in vehicle marketing, I'm surprised someone like Renault or Volvo haven't already made ISO-FIX standard fitment, and then run a big ad campaign on the back of it.
The ISO-FIX seats are expensive, and MB ones even more so, but the way I look at it is the cost of the seat is about the same cost as an A-service. Do I really want to quibble about it when it comes to safety? ...not me.
That's not to say traditional safety belt mounted seats are any less safe, as they too are very good if you buy a good quality seat. I use the belt to fasten our ISO-FIX seat into the C32, and it's good. If you give it a sharp yank though, no matter how careful you are in fitting there will always be a teeny amount of slack and you will be able to move the seat. With it on ISO-FIX mounts it's solid with a capital "ROCK"!!
Only draw back with ISO-FIX is that it is noisey!! The bracket on the seat has a metal bar, and the seat has a metal claw that locks to the bar. Whilst it's a tight fit, inevitabley there's a noticeable rattle in the body of the child seat as there is no surpression of vibration through what is effectively a solid, hard, and un-damped connection. With a belt all contact points have some play, ie the seat cushion, and the seat belt, and so the child seat is near silent.
Hope this helps.