There might be a lot more to this, both ways.
This plane is going onto N-reg, and FAA Major Mods are listed in Part 43 Appendix A. Everything not in that list is not a major mod. But it could fall under a different heading.
However, I have heard of installations like this, being just avionics, done as a minor mod, and why not? It isn't structural, and the installation is no rocket science; you basically install the kit and wire it up as per the wiring diagrams in the back of the installation manuals. Most avionics installers are basically wiremen.
The items may even be on the Mooney type certificate... in which case no approval at all is required.
EASA makes a big meal out of avionics, for no justifiable reason.
This is why, sadly, one needs to ask different avionics shops how they would do it, and use the one which is pragmatic. One needs to only follow the law as it is written; there is no need to over-interpret the regs.
A 337 for the kit mentioned, if one is necessary, could come to 4 digits.