Smitty,
The very first double rifle that I built, about 25 years ago, was a 12 gauge double rifle, exactly as you are considering. It's the perfect first project for the very reasons that you cite. I used a 12-bore gun by Neumann Brothers of Liege, Belgium, but a Fox Model B would work just fine! I use only Lightfield 1 1/4 ounce HyBred Exp Sabot Slugs in mine. These are superbly accurate and kill deer like the Hammer of Thor! Three weeks ago, scope-mounted, off the bench, at 100 yards, my first two shots from the right barrel were in one ragged hole, and the composite group was 4"!