Bearings 100%, don't even bother putting the car together until you get some.
Oil shocks also a definite.

Everything else, personally i'd just upgrade when you need it.
All the optional parts are pretty easily installed after, even the diff.
If your just starting out racing, you probably won't notice much improvement until you get better racing. Practice will improve your lap times more than any optional part will.

The differences between the 3 M05's is the pro comes with a few shiny bits, bearings and better shocks and the Ra is designed for light off road rally use so higher ground clearence