Drop-Outs - 1/3 in the first week seems pretty typical. Can be more. Six players is a good place to start, and recruit more as necessary. Beware that excessive recruiting (10+ especially) will increase your drop out rate and general entropy as players feel lost in the crowd.
Background - don't expect them to read more than a few paragraphs, but you can make more available. A 1-3 paragraph intro for everyone who joins is about right.
House rules - PBEM players have high tolerance for this IMO
Skills/Spells/Feats - dunno, just be aware that
(1) online is a different environment, don't sweat the combat positioning (unless you love spending hours crafting battlemaps for every turn!).
(2) It's much harder for players to communicate in PBEM and tactics will usually be greatly sub-optimal compared to tabletop; in fact it feels much more like the chaos of real-life combat (though my personal experience is restricted to field exercise training). For 3e, don't expect them to survive tough Paizo or Necromancer adventures at the listed levels. They should be 1-2 levels higher depending on how tough the scenario is. I'd say 50% of encounters should be at Party EL-2, not Party EL.
Edit: In practice I find that PBEM play tends to favour fighter and rogue types, which is ok as these are weak in core 3e. When running a published scenario, I'd say use the top of the recommended level spread, and cut out any extraneous combat encounters that are just there as filler. I once had a miserable time in a PBEM desert tomb dungeon crawl that was "There's a manticore/sphinx/zombies" in room after room after room. Don't do that.
