Sketchup is easy, very easy learning curve, but has limitations. You can play with it for free, but then you have to buy the plugins or version that gives you useful output. Lots and lots of libraries of pre-built parts, including RC parts out there.

Fusion 360 on the other hand and Inventor have a much steeper learning curve, but are more powerful and include CAM software built in (at least Inventor does). These software bundles take a real commitment. I found Sketchup very intuitive, kind of like Apple products in a way. Inventor/Fusion seem to be more geared towards people that have some familiarity with CAD.

Good luck