Q: Would you use hard or medium for a Rotax 503?
Medium are most common on the Rotax 503.
Q: Which Barry 22000 series mount would you use for a Rotax 618?
These mounts are specific to the airframe and not the engine. You will need to confirm the correct mount for your aircraft.
Q: Is the snubbing washer included with the mount, or do I need to buy it separately?
No washers are included. These are the mounts only.
Q: Does the price include the two halves and the inner sleeve?
Yes. Although the sleeve looks to be part of one of the halves.
Q: What is the bolt diameter?
3/8 inch.