I am looking at buying an encore 500 s&w in 15". Is the barrel threaded for the muzzle brake? Does it add any length to the barrel?

The Encore factory muzzel brake is attached via threads but it's permanently attached and not something that easily comes off. There are barrel makers that make detachable muzzel brakes for Encores.

The effective barrel length will be 1½ to 2 inches shorter than the total barrel length with the brake. Yes, you really do want a muzzel brake with the 500 S&W Mag. My personal preference is a handgun barrel no longer than a total length, muzzel brake included, of 15 inches.

If you buy the barrel and scope it, don't skimp on the scope, rings or bases. I would advise a steel Weaver base with which you can attach the scope with at least 3 rings. My scope brand preferences are Leupold and Thompson/Center.