RE: My first scratch build
I find it quicker to have every thing ready to go. Plus, you can see how it all fits (or doesn't fit) and can make adjustments as you go. I like to have the engine at the begining to make sure it will fit or if I need to make modifications to make it fit...it's handy when setting up the throttle and the plumbing while the fuselage is open too. Stoping all the time to cut a part or waiting on something you've mail ordered makes the build longer (or at least seem longer.)
My guess is it's 1/4 scale or slightly bigger.
As far as a gas engine your on your own I'm a glow guy for now.