Ive heard the same thing about not taking them apart for years, and for the life of me, after taking front and rear calipers apart and rebuilding them on all the MBs Ive ever owned, I still can't tell you why the concept of splitting them is such a big deal!

We are talking about two machined flat surfaces with one or more fluid passages that need O-rings which are usually provided in the rebuild kits ive got in the past. If not, you can replace the O-ring from about any hardware or automotive store in the world. There is even a cup to place the O-ring, and you just bolt them back together making sure they are oriented correctly and even. If you are worried about that, mark the orientation for any slight play, but they typically have 4 tightly fitted bolts, so thats unlikely.

The one thing I would not do is mix and match caliper halves as they are probably machined to specification with eachother, maybe thats where the worry comes from. Do one at a time.

With all the other things people do to their cars on a daily basis, splitting the brake calipers I would put at a #1 on the difficulty scale, about as easy and safe as replacing a fuse.

Much harder is pulling the piston out, and replacing the square cut seal. A caliper piston removal tool really helps in that circumstance, and rust makes it pretty difficult. Several times I had to give up and ended up getting a new caliper. Its amazing what you can bring back from the dead though! If you can get the piston out of the bore, and its not rusted out inside there, theres a good chance you can bring that caliper back into service!
I usually get the rebuild kits from MB for about 50-60 bucks after I know the caliper can be rebuilt.

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