Borek, According to my knowledge buffer is a system which can keep its pH approximately constant upon adding a little amount of acid, base or water. So, dilution may not severely effect pH you explained. Specially concentration ration is not changed upon addition of water.
But, dilution changes the ionic strength and increase the activity coefficient, and decrease pH. Dilution is not effecting to the pH of the buffer in any other way upon a few times dilution.
But buffer capacity is decreased and reaches to zero upon continued dilution.