6 would be acceptable, although you should kep in mind that Athas does not really hold up well to higher level magic. Athas is also notably one of the fiddlier settings-encumberance, food, water, and all the rest is supposed to be tracked. It probably wouldn't be too difficult if you had a rain cleric or water cleric, but it's worth keeping in mind.
@ VennDygrem: Sure, I can take it on a case by case basis. Just ask what you'd like. I'd prefer to keep the game at around medium op, and i'd focus on exploration, rather than trying to railroad you into a plot. (Like so many later 2nd edition Athasian modules did. Being Forced to Escort a Sorceror King was probably the worst)