You don't need rifled chokes for Brenneke slugs...they have their own "rifling". They do work well though.
You will find more guns in 16/16/rifle than 12. It's a good compromise to keep the weight down.
Also, better to get a rifle bbl for a rimmed ctg. 8x57R or 7x57R are very good choices. 9.3x72R is a big slug but a rather weak round. 74R is much better if you can find it.
Scope mounts will be determined by the bases on the gun. Some have claw bases and that will pretty much lock you into claw mounts, not a bad thing but spendy as hell if you have to get a set made up, they are not standard.
