To get the best experience with Mini-Z, my personal recommendation is always do ball bearings and damping first, before you do any motor upgrade.
Front springs, t-plate and DPS is minimum.
Then top damper and if you run lower T than 50, maybe consider side dampers and different t-plate.
Then tires of course.
So the ball diff, since I believe it is easier to setup than any gear diff.
The transmitter is way more effective to upgrade than just getting a faster motor.
Then you actually learn to drive your car.
When upgrading your motor, then also consider a new motor mount. So that the motor gets more cooling.

Personally I always true my front tires.
I run F1 (50T), LeMans (50T), Stock (70T) and Modified (39T).
I also have Porsche Cup car, a 964 that is narrow and with a RM motor mount. Both stock and Porsche uses 12T / 53T 64-pitch diff.
I trying to make a Truck also and I have AWD MA-020VE and the Buggy.

I also uses ICS on all my cars and my transmitter is a KoPropo EX-10 EURUS with the RF-902SM.
I run only on RCP, but on both sides.

To really have fun, I recommend racing with others and with a timing system. Then you can learn from others and you can learn others. You can have fun and let your work worries be left outside the arena.... hihi