12oclockhigh brought up excellent points!
I have several aircraft I created for my own use, that are based on an existing plane. I found one that was as close as it could be to the overall look of what I wanted - general shape, configuration, etc. Without 3DSMax or similar software, you can't make "parts", so it may not look 100% like what you are trying for. You can, though, change the paint scheme using far-less-learning-curve, free, software. And you can scale it up or down to be the size you want it to be. But...
Then one would dive into the "aircraft editor" of RF, to change the unseen physics of the plane - size of each component, weight, aerodynamic characteristics, etc., etc. As he noted - lots of measuring and weighing of the real thing and its components. Many fail here - the physics size and characteristics of the edited model do not match the new visual size. Here you can add/change/move components - an extra wing, new shape tail, move the landing gear, whatever. It will fly like it had it, you just won't see the change.
To date, and with a number of models, it hasn't failed that if I take the time to get the details right, it flies just like my "real" one. To the point that I use RF to test out changes before doing so on the real model ("What happens if I change the wing incidence or engine thrust a bit?", or "Is it a CG problem or something else that makes it fly wonky?")