Just a point on the amateur/homebuilt built rule
This is taken from the fullsize standard of what is classed as a homebuilt or amateur built aircraft and is the norm for permit aircraft all over the world, we would know it better as the 51% rule meaning the builder has to of done 51% of the work, to complete the aircraft, The engine, instruments etc are not taken into these figures, 51% is generally in regard to the airframe construction itself.
I have through all this in the past having been involved in certifying 3 UK homebuilt aircraft, from putting together approval paperwork, load testing to writing the assembly manuals.
These particular aircraft were really prefabricated kits, with all tubes etc pre-drilled etc but after the main build and covering you still have to wire everything up, install the fuel systems, engine, drill the instrument panel for gauges etc, which can be time consuming work, as construction time, even reading the manual all comes into account in what is classed as you doing 51% of the work.

Conventional model building i.e.taking a sig 1/4 scale piper cub kit and putting it together, fits the FAA amateur built rule the fact you didnt make the engine and the radio gear wont have any bearing on that.
Buying a hobbyking foamie and sticking together the wings and glueing on the tail wont come close, but as I said the amateur built rule line is grey, so a jet kit with fiberglass fuse and foam wings that needs gluing in formers, covering, painting etc could also comply.