I can't recommend any jewelers in Zurich, but I can recommend a few tips for buying.
Since you want white gold, be sure to ask if it has been rhodium-plated. This will make the shine last much longer. Make sure you know both the ring size for the finger, and what width of band you want. A woman's band is usually 2-4 mm in width for comfort, whereas a man's is often 6-8 mm.
These two points are very important, as once you have the item engraved it usually cannot be returned. If you're not 100% positive on both the ring size and band width, I recommend getting the ring only, and doing the engraving later once you're certain it's a comfortable fit.
Given how high prices for "luxury items" are in Switzerland, in your shoes I would seriously consider purchasing online or in Germany and having the engraving done here. Happy shopping!