FTR, ppb is not a required dive for AOW through PADI.
There is a lot of good information in many of the specialties, but some of them are crap (take it from a PADI instructor). I wouldn't doubt that you can learn many of the skills from good mentors, but without the assurance that the mentor has the ability to TEACH, you might be better off working with an instructor.
There are a few specialties that I think are essential, and some that are darn good practice and a few that are silly. Nitrox, Drysuit (especially if you may need to rent one), Altitude, Ice, Deep, Wreck, any rebreather, any and all Cavern/Cave are absolutely essential as are any type of extended range/deco/trimix type stuff. The latter are more progressions than specialties, but heed the point. Night, Search and Recovery, DPV, Photography, and PPB all have something to offer from an extended learning aspect if you're interested in any of it. Many of the other stuff like Boat, Underwater Naturalist, Multilevel, etc. can be helpful if you're really unfamiliar with the basics, but I think most divers can probably skip them and be fine.
Personally, I think every diver should endeavor to get through Rescue within the first 50 dives or so. It's the single most useful class you will take and will greatly improve your confidence and you will undoubtedly be a better buddy afterwards.
JMHO.
Rachel