My thoughts on your demands...
instant playing: every streaming service (including Spotify, Netfix,...) needs some degree of buffering; it should be kept to the bare minimum though, so it's barely noticeable, but streaming without buffer is not possible
no data connection: I don't see how that is technically even possible; if your player is in a different location from your database, there will always be data transfer, which requires a data connection; whether it's an online cloud service or your NAS at home or any other storage which is not the machine you're using... The data must go from the storage to the player somehow, it won't magically appear.
content: the idea of integrating cloud storage streaming is that you choose where you keep your content; Poweramp is not responsible for changes Google Drive or dropbox or your own NAS server make...
interface: I only use it for music, I have no experience with other media files; but of course the interface should be the same