Hello, I am keen interested in your project without no further talk i would like to tell how i'll complete this task using which tools.
1-> I will develop this application totally cloud based using (.net core 2.0)
2-> There are two ways to send message broadcasting a message or sending it individually to all the persons.
2.1-> In case we send the broadcast message then the receiver must have your whatsapp number in his/her cell phone. If not he or she would not be able to receive your messages. If the receiver has your number then we can send the broadcast message to 256 peoples only.
2.2-> We can send the message to each and every person individually and pooling the request.
3-> You would be able to choose between image,text,video content.
4-> To maintain a log file of the message sent to peoples or not we will generate a unique id for each message being sent and maintain its status as sent or not sent against it. (I will use MySQL DB).
5-> This app will be developed only for you so it will not require any kind of license.
Few other things i would like to mention
Your cell phone must have a fast wifi or data connection.
Your server must be of good quality as we need to send huge number of requests to whatsapp api and receive responses from them.
I can also setup a dedicated or vps server for you with complete configuration (FREE).
Do contact me if you are interested we have much more to talk about.