Add to that the work to make the muzzle end of the barrels look OK, without gaps.
There needs to be (usually) a filler added under the top and bottom ribs, as the are generally sweated on with soft solder and done to be a finished unit at the factory.
Chokes will depend on the wall thickness of the barrel, and whether the Smith has the appropriate reamers and taps. Plus the cost of the chokes, two sets if you want to be able to have both tubes the same choke, otherwise one full set.
Oh yeah. It may or may not still hit the same general location as when it started out, also depending a lot on the make and model, as well as the individual gun. Changing the Regulation of the tubes is...fiddly...
It's kinda a "How Long is string?" question, as it really is going to vary depending on what the expectations are out of the final product.