This really isn't that complicated, despite some of the more abstract comments about approval methods, etc. That's all fine and good from an academic perspective, but let's be practical (and helpful) here:
If you want to install the unit VFR-only, there's plenty of guidance that it's a minor alteration/logbook entry only. There are some exceptions to that (like pressurized airplanes), but those don't apply to your 182. If your A&P/IA doesn't want to follow that guidance, find a new A&P to sign it off. That said, if you have an IFR-capable unit, I think it's really shortsighted to install it VFR-only.
If you want to install the unit as IFR-capable, there are two routes: AML-STC or field approval. The non-WAAS Garmin 430/530 doesn't have an AML-STC, so you'll be doing it via field approval. There are no other practical methods for approval. The field approval process isn't hard, but different FAA inspectors will have different pet peeves in terms of the forms and wording. If you follow Garmin's manuals for the details of the install, it's little more than a paperwork exercise on the 337 and AFMS (which Garmin provides a sample of). That said, because it involves back and forth with the FAA, you're going to find a lot of shops just don't want to do it. If you're installing a 430W or 530W, the 182 is on the AML-STC and it can be signed off by an A&P/IA under a 337, no FAA involvement necessary. Garmin's install manual(s) grant broad permission to use the STC data.