You said recommend a frame for a 6'4" guy who weighs 250lb. Surly bikes have been used for RTW ('round the world) fully loaded touring and for bikepacking too. The range of frame sizes offered is decent because the fit matters.
ICE engined helicopters' chassis are made out of 4130 cromoly, it can take a few vibrations.
You might find a frame that's used?
If you are only looking for transport, why the ultra-compact mountain bike you first linked? Do you want a mountain bike, a road bike or a beach cruiser? Frame or whole bike?
Maybe look for alloy bikes from the Giant factory (Giant, Trek, Specialized) that's what I got, cheap and used, but I only kept the frame. I don't think the little two-stroke engine vibrates much if you do a decent job of building it.
(I don't find it especially cheap to build a unique experimental custom motor-bicycle that's safe and reliable either. I'm not surprised it costs more than a used mass produced scooter)