Originally Posted by Rick Hohwart

In my opinion the must-haves are:

RSD DCJ drive shafts w/hex.
RSD tuning spring set.
AE gear diff (for front).

You may or may not use the gear diff but it is a nice thing to have on hand for tuning. And if you use the gear diff you probably won't need the DCJ drive shafts.

Some other nice items that are good to have but not required:

Yodog floating servo mount.
RSD soft shock bladders.
AE aluminum screw kit.

Modifications that I recommend:

Drill a 1.5mm hole in each shock cap.
Grind the left front bulkhead and top deck so that you can get full L/R steering throw when using the forward steering post hole.

You don't need much to make the car very good so don't worry about spending a lot of money when you don't have to.