[78F350]

I'm not Rusty, but let me throw a few thoughts in here...

Originally Posted by Wendo

...
Engine control unit?

The 7.1 and 7.1.1 version Bosch Motronic ECUs are very similar to the 7.2 that is in the 2000-2002 Boxsters. For these, it is mostly a matter of matching up the engine harness and Audi ECU to the existing harness on the Boxster. Wire-by-wire without any clear step by step guides, but lots of info online and in manuals that it can be deduced from.
You have a 1997-1999. It has an older version ECU (Motronic 5.2.2) and a cable throttle. That makes it even more challenging to swap a 40 Valve Audi V8 (roughly 2000 - early 2003), but easier for the 32 Valve ABZ engine. The ABZ is cheaper, less complicated, and more reliable. Same idea applies to using the Audi ECU that comes with the ABZ....

Programming?

To run, no. To run well with no MIL, yes. If you use the Audi ECU, you can have a pro do the programming, or spend some time learing about Nefmoto or similar hack/tuning software.

Does it need its own immobilizer unit?

2000 and prior US models, no. After that, the immobilizer can be bypassed or programmed out.

This is sounding more like a massive endeavor.

Yes.

Is there a short sum of all this?

No.

Can you advise a source to learn the electrical needs of the Audi?

Some sources:
Bentley Publishers - Repair Manuals and Automotive Books
NefMoto - Index
motorgeek.com
https://www.google.com/search?q=audi+v8+swap+wiring&ie=utf-8&oe=utf-8

I am planning an operational stand with cooling for engine testing before ever touching the car.
The Detroit engines can have stand alone control.

Check out this dude...

Are the Audi 8s really electrically heavy?

Relative to what? More than the Honda's I used to swap, less than some others I'm sure.
If you know Detroit, You will probably find Detroit easier. I know nothing about GM and Mopar engines, so the Audi is not more difficult for me to learn.

Pay attention to what Rusty said about flywheels. Most Audi V8s in this year range did not come with manual transmissions in the US. A 10-bolt flywheel and clutch arrangement for the ABZ is not as easy to find as the 8-bolt for the later engines.

If you want to use an Audi V8 engine, you can still use a Detroit throttle body and choose your own custom engine management package.