The Republic was never intended to be invaded by the Empire. It's clear from the very first Star Wars novelization-as well as early scripts-that the Empire was the Republic.
What did change however was the villains. Originally, the Emperor was actually just a reclusive puppet ruler and the moffs were in charge. But apart from that, this outline pretty much is the prequel story, right down to the commerce part.
Here's an excerpt from the intro to the Star Wars novel:
Another galaxy, another time.
The Old Republic was the Republic of legend, greater than distance or time.
No need to note where it was or whence it came, only to know that...it was
the Republic.
Once, under the wise rule of the Senate and the protection of the Jedi
Knights, the Republic throve and grew. But as often happens when wealth and
power pass beyond the admiralble and attain the awesome, then appear those evil
ones who have greed to match.
So it was with the Republic at its height. Like the greatest of trees, able
to withstand any external attack, the Republic rotted from within though the
danger was not visible from outside.
Aided and abetted by restless, power-hungry individuals within the
government, and the massive organs of commerce, the ambitious Senator Palpatine
caused himself to be elected President of the Republic. He promised to reunite
the disaffected among the people and to restore the remembered glory of the
Republic.
Once secure in office he declared himself Emperor, shutting himself away
from the populace. Soon he was controlled by the very assistants and boot-
lickers he had appointed to high office, and the cries of the people for justice
did not reach his ears.
Having exterminated through treachery and deception the Jedi Knights,
guardians of justice in the galaxy, the Imperial governors and bureaucrats
prepared to institute a reign of terror among the disheartened worlds of the
galaxy. Many used the imperial forces and the name of the increasingly isolated
Emperor to further their own personal ambitions.
But a small number of systems rebelled at these new outrages. Declaring
themselves opposed to the New order they began the great battle to restore the
Old Republic.
From the beginning they were vastly outnumbered by the systems held in
thrall by the Emperor. In those first dark days it seemed certain the bright
flame of resistance would be extinguished before it could cast the light of new
truth across a galaxy of oppressed and beaten peoples...
From the First Saga
Journal of the Whills
As for the whole Honogr/Noghri stuff, on which the timescale for the CW was based originally, that's actually explained quite well in one of the Clone Wars comics. Hint: Different planets have different rotations.
