It looks like Greg Sievert started his project in 2009. There have been changes in cost and technology since then.
Here's a low cost, short range conversion:
https://www.youtube.com/watch?v=gcRGqd8VSms
This one has batteries in the engine bay and rear. Not much information on the conversion -- a lot of the video is about how sexy the car is.
https://www.youtube.com/watch?v=VlDXqgaJ5lQ
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Present: 1990 240GL saloon, 1992 240 estate
Past: 1988 240GL; 1971 144DL; 1972 145DL