Been changing spark plugs for Honda, Toyota in the past and it was really easy to change it. How about on ML 320? Do I have to have special tools and knowledge to change it on ML 320?

Best Regards,
Oat

Not too bad, but you will want to remove the coil packs and set them aside.
Unbolt the torx screws holding them down.

When removing the spark plug wires, be sure to note the locations,and be careful removing the wires from the spark plugs, in most cases, you will damage a couple and the car will run like crap when you are finished until you pinpoint which ones you screwed up.

Scared yet?

Those wires are a pain in the butt, but the miss that is produced, if in fact, you do damage some wires are easy to pinpoint and fix.

You should be fine with a standard swivel spark plug socket.
And for the wires, I use a generic 45 degree bent head spark plug tool with rubber ends where it grabs the wires, and the standard mercedes spark plug wire removal tool.

the rear wires are tight and those are usually the ones that get damaged due to clearance problems and general frustation.

Take it slow and don't rush the job.
Pay close attention to locations.