Well, there's the Playing the Game and Talking the Talk boards here at ENWorld which are nothing but pbp.
I've been running a campaign via message board since January 2006. I'd say that, as a rule of thumb, each hour of normal play translates into about a month of pbp, and that's with regular posting. If you have slackers, it can take even longer.
As a result, the focus should be less on slam-bam action (although it's D&D, so some of that is almost always appreciated) and more on roleplaying, since people can take advantage of the time between turns to post long discussions between characters and the like.
I recommend taking out some of the randomness of the game and using point buy or Invisible Castle, which records rolls for later. Likewise, when I do it, I roll all the dice as DM, but Invisible Castle is some folks' preference as well.
Battlegrids are difficult -- but not impossible -- to do, so you may want to go with a more narrative style adventure, but if you have a FTP space, an image editing program and a bit of patience, you can make battlegrids work.