FI Options
Your options basically fall into three categories:
1) Retrofit the original D-Jetronic system that was in the 3.0CSi. This is generally not advised, as D-Jet components are getting more and more difficult to come by.
2) Install an L-Jetronic system with Lamda (oxygen sensor) from something like an '81 E12 528 or a non-Motronic 633 or 733i. The intake manifold has a very similar look and feel to the D-Jet intake manifold, so it doesn't obviously look like the intake time-traveled in under your hood. The retrofit is fairly straightforward and doesn't require other sensors such as bell housing or crankshaft (see below). Well, ok, I lied, there are other coolant sensors, but that's about it. You can keep the original ignition setup or update to the dizzy with electronic ignition that comes with the later L-Jet system. This all can be done without pulling the engine from the car. This is what I did nearly 15 years ago and I've been very happy with the conversion. I can let it sit for six months, then hook up the battery and turn the key and it starts right up and idles fine.
3) Install a full Motronic setup. If you happen into something like a 3.5 Motronic engine with manifold, wiring harness, and all sensors, this is an upgrade that many folks have done, but it's more involved than the L-Jet adaptation. Some folks have grafted on an L-Jet intake manifold to replace the Motronic manifold so it looks closer to period-correct, which as I understand it requires modification of the fuel rail. Depending on the year of the Motronic system, it will require either a crankshaft sensor or a bell housing sensor. A late 3.5 engine with a Motronic setup will get you more oomph, and better gas mileage, than the comparatively antiquated L-Jet setup.
Any FI retrofit will require a gas return line.
All of this is a fair amount of work, and may be best approached as part of an engine refreshment (ie, maybe you don't want to screw with it until your engine needs to be rebuilt, then do it all).
