Personaly, the first to get are.......
73814 - Woven Graphite Main Chassis
73454 - Graphite Suspension Arm/Rear Hub Set
On that you can build a reasonably competitive car.
After that a woven chasss plate would be the go.
Im a bit behing the times......just got my HPI full length upperdeck and HB front and rear shock towers! Very nice.......then i cut up the bulkhead covers. Looks very nice!
I dont see what people complain about, the pro3 is a realy fast car and handles very well (with options).
Hope this helps!