I'm partial to twins, and I like your project.
The Beech Baron B55 in my avitar was 50" span and weighed 4.25 pounds and powered with twin OS 15s. The equivalent of 15s or better in electric would be my guess for your power. An 8" diameter prop was scale for the B55. Was an excellent flyer and was sold years ago.
Good luck with your project.
feihu