Scope questions

I just finished putting my optic-less upper together for my new 18" barrel build. I know need an optic.

Its a little bit daunting all of the various options. So if the hive brain could give some help with these following questions, I'd be appreciative.

Targeted range: 500~600 yds, primarily used for target shooting.

1) pic rail scope mounts or canteliever mount. Like what would justify the canteliever?

2) Glass. Like what magnification should I be looking for? Im thinking maaybe 5-700 range, but can't decide on magnification.

3) what's an appropriate range on a RDO and magnifier combo?

Lastly, not an optic question. But do BCGs play a role in accuracy? Like should I get an expensive one to help with any accuracy issues? Or just get an Aero, Griffin, or BCM BCG?