If you've never changed the timing chain on a M119 engine - DO YOUR HOME WORK before you start. If your planning to roll the new chain in, you'll need at least 3 people to help to keep the chain on the camshafts. There are numerous tips and steps regarding this job that the pros use. Needless to say, this is a major job and I would not recommend it for a DIYer. I'm speaking from experience.
I'm a seasoned DIYer, but my next chain replacment on a M119 will be taken to the shop. I've done chains on other models of cars and nothing compares to the M119.