First, this watch will not be solid 18 k gold. Not at that price point. Guaranteed. Apple already said the metal is mixed with something to make it harder than gold, hence, it is not pure gold.
Second, I am a watch collector. I have more invested in watches than I would care to mention. The watches I collect however are just that, investments. They often hold their value or many gain value over time. A consumer electronic like this will not. It will depreciate just like any consumer electronic will. And it will probably depreciate a lot more than the base watches which will all do the exact same thing as the more expensive version.
Will people still buy it? Yes, because people are foolish with their money.
18 carat gold is 75% gold 25% other metal. 24 carat gold is pure gold. The casing of the edition watch will be 100% 18 carat gold.