Ask for an X-Ray. It can tell for sure. All the dentists I've gone to would do it first thing on the first visit as a record. (A lot of murdered victims are identified by dental records.)
I would have the cavity fill as soon as possible because I can't stand the thought of having something rotting inside my mouth. When you get to a point that you feel the pain, you might lose the teeth or need a root canal, something you can avoid if you fill the hole when it's still small. And yes, cavity will cause bad breath and you don't want to lose any friends because of that.
What is 種稙牙手術? I don't know if it's the same as crowning. I had the mercury filling that was done 15 years ago remove recently. The hole was so big that my dentist thought I eventually might need a crown but for now he would do the filling and see how it goes in a year or so. I think crowning is like a last resort. You don't want to do that unless you absolutely have to. Most dentists offer both mercury and white filling. Mercury is cheaper but it's not safe. I would go for the white one.
no 蛀牙 also 會有口氣 ,咁點算
If you are healthy, don't have cavity, and floss everyday, you shouldn't have bad breath. It's very important to floss everyday but it seems that Chinese are not really into this. I can't get my parents to floss and most of my Chinese friends don't have this habit either. I read it somewhere that Asians are more prone to gum disease because of the poor flossing habit.