Hi!
My DC-3 fuselage is first covered with 25g glassfiber using professional 24 hour Epoxy resin...not the hobby stuff variety.- There is a huge difference!
The epoxy I use get glass hard after 18-24 hours,but is easily sanded.

After I covered the fuselage I spray painted it with Sikkens Autocryl 2 -part primer filler, then sanded it to a smooth uniform surface so that the wood grain would not be seen through the plastic covering. Then small stripes of Chrome Oracover was placed on a glas surface and wet sanded with 1200 grade wet/dry sandpaper.
Each strip was then placed on the fuselage and fastened with an iron.

On places with dubble curvature such as the rear most end on the plane and on the wing fillets , I used pieces of aluminium duct tape (first wet sanded on piece of glass).

Oracover fastens very good to a sanded glasfiber surface, soo you don't have to be anctious that plastic film loosens over time.
The only Oracover film that does not like to be strech repetadly is the "silver" variety.That's my experiance!

I would stronly advise you to put in a wing spar in the wing and not only rely on a glass covered surface. Using a thin spar is both stronger and lighter than a glassfiber covered wing and will also give a more ridged wing, which is good.

OS FS .20 will give plenty of power and is a good choice I think.
If you want I have scale, carbon fiber, 3-blade, "Hamilton standard" typ props for the OS .20 and .26. which will perform equall or better than the best 2-bladed props ( RAM or APC.

It's a very common misstake by newcomers to twin engined planes to over motorize them! Saying: "Better to have much power than too little...I could always throttle down" Sorry to say but saying so only reveals how little they know about twin engine flying and how to build.
It's much better to use as small engines as possible and build as light as you can. That way ,if everything works out as you have planned, you will have a smoth, easily flown airplane that behaves like the full scale plane in the air, not like a pattern plane!