Circuit City put keyless entry and an alarm system into my '79 300CD a couple years ago. It was really easy for them. I haven't read bill streep's writeup of it, but all it takes is one small actuator that pushes the rod attached to one inside door lock. The rest of the locks work by the vacuum system. When my vaccum system blew out (due to age, not the install) I added the actuator to the passenger side door so that both doors still had power locks. Maybe a twenty minute job. I would recommend buying an actuator and install it yourself, and then run the wires under the dash, and let the alarm system people connect it to their security system.
'86 560SEC AMG