The Citroën SM is in one respect just like any other car: it needs proper maintenance! It differs with other cars, that it needs particular attention and maintenance which is typical for this car. Don't forget that cars which were built in the seventies, need maintenance more frequent than nowadays cars. But then - you are driving an exceptional car with unparalleled pleasure of driving it.
The workshop manuals are all on line, so no excuses- get them. You can print the pages you need if you take the car to a workshop rather than do the work yourself.
Basically it needs attention and maintenance just as a Citroën DS, so for the brakes, suspension and hydraulics in general, you can visit a good DS specialist or shop. However, the engine needs special treatment, which can be done by Maserati specialists. Some specialists are a true SM specialist, but there are only a few, so beware!