Will 250cc with its diameter of 12.3cm (Mentor round) be enough to fill out my chest? (Photos)

Thank you for the question and pictures.  Based on your photographs, I think that you are starting at a good place and should have a very nice outcome with breast augmentation surgery.  I also think that your goal photograph is realistic understanding of course that goal photographs are communication tools not guarantees of exact outcome.
My best advice: more communication.
It is definitely worth spending more time (prior to the date of surgery) with your plastic surgeon to discuss further.  As you know, it will be important for many reasons, that you feel comfortable that you and your plastic surgeon are on the "same page" prior to proceeding.  Show your plastic surgeon what your goals using as many visual aids as possible.   As you can imagine, terms such as "natural" or "fake" can be interpreted differently by different people. This additional communication will go a  long way when it comes to helping alleviate your anxiety prior to proceeding.
 
 Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery (regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. ***Ask to see lots of examples of his/her work. Sometimes, it is necessary to seek several consultations before you feel comfortable about your choice.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural or modest outcome” or "C cup” etc may mean different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, best not to discuss your goals and/or judge the outcome of the procedure performed based on achieving a specific cup size. The use of computer imaging may be very helpful during the communication process. The use of in bra sizers may also be helpful. In other words, use as many “visual aids” as possible during the communication process. I encourage patients to meet with me as my times as necessary, to feel comfortable that we are both on the “same page”.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery. I generally select appropriate breast implant size/profile after the use of temporary intraoperative sizers and viewing the patient's chest in the upright and supine positions.
 
I hope this helps. Best wishes for an outcome that you will be very pleased with.

The optimal breast implant size depends on many factors that can only be determined with an in person examination. Chest wall measurements, skin elasticity and the desired results should all be considered in choosing an implant size. The surgeon's skill and technique will determine the success of the surgery. You should understand that an implant size does not correspond to a bra size, because each person has different breast measurements. An implant of the same size will give two different individuals with two different results. This is because one may have a smaller chest wall measurement as compared to the other. Thus the best course of action would be to closely discuss in detail your expectations and desired results with your surgeon before choosing an implant size.