The only difference between the 1983 and 2004 versions of the rancor fight (other than bad color timing in the 2004 DVD) would be the re-compositing and removal of matte lines in the 2004 version.
And I dunno why the prices work our that way in France, but any 2-disc release of the original trilogy contains both the 2004 DVD and the 2006 GOUT - and it's also the only way to get the 2006 GOUT.