I have not built the GP spitfire but I have done several smaller planes in fabric and paint that worked pretty well. I would suggest building like you were going to cover with moneykote, no extra sheeting. This keeps it reasonably light or at least within its recommended weight range. Cover it in a fabric then paint it with spray paint, (I dont recommend luster kote for color), something like 21st century or rustoleum. Rustoluem is what I have used many times and I've seen camo colors at the hardware store recently. I would suggest clear coating the rustoleum with luster kote, thats the only luster kote I would use. 21st century is a pretty good paint, much finer pigment than luster kote and rustoleum. I dont know about mechanical retracts but the previous suggestions make sense to me. I can tell you from experience that a small plane built heavy is gonna be pretty disappointing. Dont even consider glass on a .40 sized. Im usually leary about doing any glass work on 60 sized planes, thats an acquired skill that comes with experience. It can be done though. If you build to sport scale standards and concentrate on the paint job I think you'll still be pretty satisfied with the outcome and still have a decent flyer.
Just my opinion.
Edwin