Maybe do your sister/BIL's car first?? What happened?Do you need my address, so you can ship me your car, and have all the parts drop-shipped? I'll make the time.
For anyone with a Djet, who really wants to abandon the original Bosch system, Barri on Peachparts did a MS conversion a few years back. He documented it here:
Converting a 1974 450sl to Megasquirt
Well worth reading. Then go and read the rest of the stuff on the net - there is a lot of it. This a good place to start: DIYAutoTune.com Megasquirt Kits / Assembled Engine Management Systems, Wideband o2 Sensor Systems and tuning products
As Daryn said, be prepared to spend a lot of time on tuning. Not many have done DJet MBs, so you could be on your own.
One problem I see, is that many are striving for fuel efficiency. But, our old V-8s were designed to run rich. Some old mechanics warn not to try and run close to Stoichiometric. Could damage the engine.
After researching MS, I decide to instead try and get my D-Jetronic system to run as it should. The addition of a full time open loop Wideband O2 sensor and AFR gauge really helped. If you don't know what the mixture is on a Djet, you are dead in the water. Same is true for MS.
Only reason I would go to MS, would be if I needed a new ECU or MPS and neither of these could be obtained new or used. Others however, find it a neat project, so go for it if you like that sort of thing!