Drupal module powered by Wikitext
First recommendation: Just use MediaWiki for the whole thing. It's not as nicely modular as Drupal but it has a lot of facilities. And as a bonus it's likely that upgrading will be easier (Dn->Dn+1 has always been a pain to me, MW upgrades have always been very smooth for me).

If you still want to use Drupal, I would use MediaWiki's parsing. E.g. fetch articles through index.php?action=render&amp;title=Page_Name. It's probably best to simply fetch data when needed but with a long cache time (e.g. 1 week) to avoid Wikipedia blocking your server's IP.