In my opinion, the best alternative is to use a slight excess of HCl but control the amount of nitric added so that you only use whatever is needed to dissolve the gold. No more, no less. That way, with no excess nitric, the gold will directly precipitate with the sodium metabisulfite without the need for sulfamic acid or soda ash or urea or anything else.
In practice, it takes about 120-130ml of HCl and 25-30ml of HNO3 to dissolve 1 tr.oz. (31.1g) of karat gold. To make sure there is excess HCl, I would start with about 140ml per tr.oz. Add the HCl to the gold and heat it. Don't boil. When hot, add about 1/4 of the calculated amount of nitric. A reaction (fizzing, brown fumes, etc.) will be seen. When the reaction dies down, add another 1/4 of the nitric. Repeat. After about 3/4 of the nitric has been added, reduce the size of the additions. When a small addition of nitric produces no reaction, add no more. It may take a little more or less nitric that what is calculated. If you do this right, there will be little or no excess nitric and, therefore, no soda ash, sulfamic acid, etc., will be needed. Try this on small amounts until you get the hang of it.
It is good to have some sulfamic acid on hand in case you are heavy-handed with the nitric. For this chemical, I would find a company that sells electroplating chemicals and supplies. Sulfamic acid is commonly used to make up nickel sulfamate electroforming solutions. In any case, I would never use soda ash.
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