The W126 factory alarm is pretty good, though it has no shock/motion sensor & remote lock/unlock. A popular aftermarket product to add these features, that piggy-backs on the existing factory alarm, is the Ungo Box -- now owned by Clarion Electronics.
Taking it to an MB-competent installer is the key. I ripped my Ungo alarm out after several things had shorted/rusted from a bad install by the PO's install shop. Insist upfront on insured and warranted work -- I have dealt with a radio installer that would not replace some panels they broke. Best of luck, Andrew Seidel.
85 500SEC Lorinser
83 300D Turbo