I am pretty sure the good ones use machined forgings. Ruger used investment casting - their claim to fame across the board.
What would your Dad be using it for? Be advised the English "game guns" are not designed for heavy loads. That's where the American style doubles like the Winchester 21 come into play. In either case workmanship and balance play a huge part, and engraving is frosting on the cake.
I do not think you need to know how to weld, but silver soldering would be good, even though many very good guns have been built using soft soldering. If you use a monobloc for the barrels it should be easier than other set ups I would think.
For an English style, my advice would be to buy a decent Spanish gun, copy it with first class materials and workmanship, and then sell it. For an American style, a Winchester 23, Browning BSS, older Fox Sterlingworth, or Parker should do. If you come up with a way to make a REALLY good one, in 12 gauge with 3" chambers, with a longer water table like the Winchester 21, and sell it for less than $5000 in quantity, you could get rich.
I wish you luck.