Prowler555
Have you designed, built and flown successfully a plane?
The problem with your specification is it will not only require some detail understanding of the strength of materials but also considerable building skill to put the design features into reality. I would also expect it would be an iterative process proving each design feature.
A quite voluminous 1.8 kg 'payload' within a 1.2m span sounds like the specification for some sort of military UAV rather than a practical model RC plane.