first off welcome back to the hobby... I too started about 15 years ago...and just got back into it about 5 years ago..and my driving stilll sucks...
RcModel now has the graphite shafts...I got mine from RainbowTen in Japan.. along with the Ti front/rear input shafts.
the reason for replaceing mine is that is make for a lighter drivetrain when racing stock...for mod..I'll stay with the aluminum shaft... since you're running brushless..I suggest you stay with the aluminum shaft...the graphite shaft may break
for the beveled gears...only 1 version is available right now...I kinda over-cooked mine to more than well-done...so I purchased extra sets.. a set to install and several more for back-up reserve.
On the Delrin Diff...haven't seen it available yet...but I'm cirtain it will be available sometime soon either by Square RC or HB. advantage...hmmm tough one...the stock graphite one sometimes can break under hard acceleration...the delrin is more forgiving..lighter, more durable & makes for a smoother diff. I had one on my Pro3..big difference over the graphite one.
The Servo arm is made my Ofna...Tower has them..or try your LHS
and the spring perch..also got from Tower...RcModel should have them as well if you're going to order from them again.
The front upper graphite bumper brace is from the HPI RS4 3 nitro. got that from Tower...but all sold out and on back order..
Ceramic balls...very light and makes for a smooth diff. As mentioned earlier there was no difference in hardness between the ceramic and stock carbide balls. Just stock up on extra diff rings.
Feel free to PM me or just post here... eveyone here is friendly and won't bite your head off for asking so many questions... we're all here to have fun and help each out...