Buffer to have ( 5V-->15V) with DAC conversion
You can power the CD4049 from 15V and directly drive into the power stage, but you will still need voltage translation from the 5V domain to the 15V domain prior to hitting the input of the 4049 since you would not be able to overcome the Vih of the 15V 4049.  A transistor + resistor may be able to work for your application as the voltage translation stage pending how quickly you need to switch.

Something like this?