[EDIT]I've looked at the modules mentioned above and both would require some conversion, "Expedition" prohibitively much I believe. The adventure of Skip's I'll keep in mind, but I would prefer to choose something published under the 3.5 ruleset.
I'm relatively new to PbP (6 months or so), but I well understand the nature of the beast. CotSQ (I've played through it in its entirety) could take a year as a tabletop game. I'd be surprised if PbP didn't take 3 or more. Something a little less expansive is probably in order. Underdark adventures are a very good fit for this level range, though, so I'll check out the suggestions listed so far.
Are there any other ideas? Planar exploration? Does anyone have opinions on the nature of the game, i.e. lots of combat, lots of dramatic interaction?
Also, feel free to begin posting character ideas, etc. The longer I have to work with you on any details, etc., the better chance of getting you into the game (no matter the setting).