Just goto any pick-n-pull. Look for any mid-80's gasser and you should find 2-6 vacuum solenoids per car for engine emissions. They are really easy to setup, just wire/tube it to vent vacuum to the sol as if it were turning on the ignition of a gasser. If you hit the kill for the remote, the sol will return to neutral, sending vacuum to the shut-off.

Most come with a delay option, just make sure it does before you lay down the $$$.

A remote start won't be much use on an OM617/603 in winter. You would have to let it idle for 2 hours before it gets to 60*c.

To me, a remote door lock/unlock would be much more useful. It's been done before to a W123. I've even got the electronics, bracket, bent pushrod, and wiring. I just have never got around to installing it.