RE: Which motor?
There are alot more factors to consider than weight and wingspan. The type of airfoil, application (ie. trainer aerobatic ect), gearbox/propsize ect.. Since the plane is being custom built maybe you can go the other way around and choose a motor/prop and build the plane to suit it. Using other planes that have proven succesful and are in your size range is a good way to start and then model your plane the way you want around those parameters. Learn the general rules of watts/lb for your application so you don't end up with a dog or something that shreds your firewall when you max out the power.