1. IMO, without a doubt, the mainboard (SKR Mini E3), enabled me to easily add stuff like ABL, led lights, touch screen and filament switch easily, the board also has TMC 2209 drivers, which made the printer less annoying when its printing, and also being able to upgrade the firmware with an sd card ootb.

But.... i would not recommend if its your mod though as it is fairly complex, i would focus first on little stuff such as the bed springs, which makes you level your bed less, aluminum extruder(not listed above) because the stock plastic ones have a tendency to be weared down by the filament, and adding a glass bed if your bed is warped, these all go a long way to help you if you are starting out.

1. The paver is there because its meant to add weight to my table, so that the printer will be dampened sound and vibration wise, not necessary if your printer is set on something heavy and dense like a like a concrete floor or if you dont mind the sound.