Hi,
Its up and running in my daily driver, a 1985 500 SEL. I transplanted the engine from a 1988 560 SEC.

You need the K jet metering head/fuel distributor from a pre 9/85 500 SE/SEC/SEL at the minimum, I would get the whole intake manifold assy complete and use it as it is, apart from swapping the larger 560's throttle body. This setup uses no computer or control box, just purely mechanical.

As far as the ignition distributor goes, I also used the pre 9/85 setup. You need the distributor itself, and also the ignition module/amplifier on the passenger side inner wing. I used the more powerful 560 ignition coil. And depending on if you need the hydraulic pump, get the blanking plate from the distributor take off as well.

You could probably get the old contact breaker style ignition setup from a D jet M117 car if you really wanted it simple, but I wouldn't recommend that with LPG, you want a good strong spark.

I reused the idle speed computer that was already in the car, but the engine doesnt need this to run, you'll just suffer from a high idle without it. A screw type valve could take the place of the idle control valve, and it could be manually set.

Hope this helps.