Before you get a million people telling you need stuff....
The kit out of the box is capable of winning the worlds.
The only two things that I think you can say are CLOSE to being required (but really aren't):
Hex conversion (makes it easier because you can borrow tires from others if needed or to try)
Aluminum steering rack ( stock steering servo saver deflects too easy and you lose steering. Either glue it or replace with aluminum)
that's it.
The big bore shocks are nice, but not needed. Eventually switch to them so that you can use current setups, but this can wait for quite a while.
The slipper (Avid or VTS) will not be noticed by a newbie
As far as spares... I would keep a set of front and rear arms, caster blocks, and bulkhead handy.
One thing you will notice with the 4.1 is that the plastics wear faster than some of the other cars. This isn't all bad. The slop in the car actually adds traction on lower bite surfaces
Once you get to a point that your driving is suffering from parts wear, then you will want to change them over. Generally speaking, you will do this a few times a season. You can avoid this by running carbon parts, but they can be more brittle in lower temps. For a newbie, I would just use the plastics.
The only other thing I can say is that it's just like onroad, if your tires aren't right, no amount of tuning will help.
Overall, the 4.1 community is pretty helpful here so keep yourself active here and you'll enjoy the experience