The motor size will depend on the weight of the airplane and the style of flying you want to do. Most electric motors will come with a mount some will have the prop adapter other price it separately. Remember you must use an electric prop NOT a prop designed for glow engines. Hacker has a good guide for motor selection here:
https://hackermotorusa.com/resources...ication-guide/
You can buy from them or find a less expensive equivalent from another supplier but Hacker makes excellent electric motors.