What else do I need to complete?

[LRRPF52]

This is a hunting set up I take it? That's a 1" scope tube, so remember that when looking at mounts. I would look for a mount/ring height that is going to match your cheek weld to eye height, and keep it lightweight.

You need a Lower Parts Kit
Ejection port cover assembly (if not already installed in your Mega set)
Forward assist assembly
Rifle length gas tube

If you have the ACE rifle length Skeleton stock, you can use an AR15 rifle buffer and spring, or a JP Silent Capture Spring with spacer.

You also need a charge handle. I like the BCM Gunfighter series, as well as the Rainier Raptor ambi CH's.

For single stage, I use JP triggers and speed hammers. The Timney's feel great for a single stage as well.

Then you will want a pistol grip that fits your hand well, and most importantly, sets your trigger finger up for a straight pull to the rear.