Is 280ml Breast Implant Size Too Big for Me?

You have provided very detailed information but the size and shapes of the different manufacturers may vary.
In general, 280 is a modest sized implant for the average body type and should suit you well increasing your breast size to a full C (on average).
Because you are 25, it is extremely likely that you will need additional surgery in any case. So it is not likely these will last you lifetime.

Choosing the correct implant for you really depends upon an evaluation with a doctor in person and a discussion of your concerns.  It is difficult to suggest in thhis type of forum.

Hello,
From the way things sound, the 280 cc breast implant size sounds within small to moderate. I can't be more certain without examining you, of course.

As a double board certified plastic surgeon, I would recommend choosing the right plastic surgeon to do your surgery. In order to achieve the look you want, it is best to use a sizer to determine what size implant would be best for your body type.
Internally, you body is very different. Your chest cavity may be concaved, or it may be convexed, so you may need a larger or smaller implant size. Also, keep in mind that you may need different amounts of cc's for the right implant and the left implant.
As long as you let your surgeon now that you don't want to look top heavy, or to go too small, they will be able to decide how many cc's you would need to give you the look you want. Show him/her pictures of what you want done, what's too big, what's too small.

I never recommend an implant strictly based on numbers.  Yes, measurements are very useful to determine implant sizes.  But an examination by a board-certified plastic surgeon and a consultation to discuss your desired size and shape are imperative.  Best of luck.