Absolutely no reason to get the Calmini lift kit. All the additional parts are not needed, but can be purchased separately from Indy4x at a later date if you want.

But yes after a spring lift and torsion crank (2-3" suspension lift) a 2-3" diff drop is the next most important thing. After that adjustable lower links would be #2 most important (to correct pinion angle and decrease driveline vibration on the highway).

Then after those mods the optional bits in the Calmini kit are next, but not required.