wrote:
I'm in the process of looking at buying a plane.

One of the options I've considered is buying a used RV6A or RV7A
or maybe even an RV10. (Flying RV10's are Still a bit pricey)

I want an IFR capable aircraft and I want all flight critical instruments, radios etc...
to use certified components.

If I find a used airplane I like but want to add or upgrade the flight instruments and/or avionics

what is the legal or regulatory equvalent of a 337 or STC for a homebuilt?


Your builder's logbook. Period.
Anybody can do anything to a homebuilt. Only if it is considered a
major alteration, depending upon the particular set of operating
limitations it was assigned, you may have to put it back into it's
original test area and fly off 5 hours, and make a logbook entry.