I think her cravings for sugar are completely typical for all kids. I even had a brief entrepreneurial phase in middle school where I would bring in candy to sell to other children...there was always demand.
You don't give a clear indication about the quantity of candy that she buys, so it's hard to tell how unhealthy it might be, although you assert that she is physically fit, so it must not be awful.
I think the system where she can use her money to buy what she wants makes perfect sense. Of course you give her that money, so it's up to you how much to give her, and if she is buying too much candy with it, you can give her less, or you can make her have to work harder for the same amount of money. Or you can parse it out with her, and say she can have a dollar (or whatever amount makes sense for you) for whatever she chooses, and the rest is going into a "big toy fund" or something like that so she learns the value of savings too (and doesn't feel like she's getting a pay-cut).
The other thing I'd recommend is, since she's got such a sweet tooth, you should make sure to have a lot of fresh fruit always around, and make them freely available to the kids. Whenever they want an orange, it's fine.
If there are rules about candy consumption (not before dinner, no more than 3 pieces a day, etc.) then that's fine too, as long as they are consistent. Then follow those rules, and try not to add in extra little punishment in the form of lectures every time you catch her eating candy (even when she's following the rules). Doing that can give kids a complex over food, make them feel like they have to hide the fact that they're eating it, and make enjoying a well-earned candy into a shame-filled experience.