Is the Version on sale by GOG the original DOS Version (SCUMM/Dosbox?), or is this the newer Version that was (is?) for sale at Wyrmkeeps site for $20??

Please advise - should I buy it as owner of the old boxes (MS-DOS, CD-32 and Amiga ECS)?

Well, it is a Windows port, but otherwise identical to the original as far as I can tell.

The Wyrmkeep update fixed a missing voice poart in the intro in ENglish version, exchanged Dreamers GUild logo with the one from Wyrmkeep. I think, some other glitches were fixed. I can't finde the information on Wyrmkeeps site anymore, but ScummVM wiki mentions something about digital music:
http://wiki.scummvm.org/index.php/Inherit_The_Earth