Brian,
The unit I put on my 91 300D was a Python 1400XP. At the time, It was the only one I could find that did remote start on a diesel. I'm sure there are others. It is a keyless entry and remote start, but no alarm. It will however arm/disarm a factory system if your car has one. Mine doesn't. It is a full feature system that will require some additional relays for some functions like interior light control. A starter disable feature is available that requires a relay also. I didn't put that option on my car. I don't want ANYTHING that will keep my car from starting when I want it to. In our shop, we saw many, many cases where a car wouldn't start because of an alarm systems that had starter kill features. The systems would malfunction and require a tow. We would get the vehicle after someone else had replaced the battery, starter, alternator, etc.
The Python is a border line DIY install. What I mean is someone without any electrical knowledge may have a hard time doing it. You will need to wire relays, etc.

Paul

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84 500 SEL (307,xxx miles)