Hello to you too.
Get a used Colt/Sauer Drilling in 12/12/30-06 and try it out as is. If, after familiarization and testing you still think you need shorter barrels and mobile chokes, contact a good gunsmith familiar with Drillings and disscuss these modifications with him.
I prefer German claw mounts but EAW or similar side pivot mounts are also an excellent choice. As for the scope, I would try Burris Euro 1.5-6x40 with illuminated reticle.
There are many insert barrels (and cartridge adapters) available so the possibilities here are almost unlimited. But I would try to avoid overdoing it as the gun can get too heavy and too complitated very quickly.
Regards,
Jani
