quickie - started reading up about megajolt.

is this needed if one has megasuirt?

does this replace EDIS (whatever that is - will read up on that!)

(application would be 2.0l zetec engine using original manifold and injectors)

megajolt is ignition only. iirc megasquirt also has megajolt built in.

EDIS is the coil pack etc that you also need with magejolt to get the ignition working. Megajolt is therefore only the computer that fires the coil packs. In std form it is 2D management. If you add a throttle position sensor it will be 3D. You also need the crank wheel (36 - 1 teeth) and sensor.
Megajolt does away with the std dizzy and points etc so in summary makes your ignition fully programmable electronic.

Megasquirt controls fuel injection.

Very well described Mark.
One thing you forgot...
Electronic
Distributorless
Ignition
System