I already have original gta V game files. If I purchase Gta v Digital Download CD Key from here
https://www.kinguin.net/product/113083/grand-theft-auto-v-rockstar-digital-download-key-1428435386-65307/
Possible to play online using this cd key? or I have to re download it from Rockstar Games Social Club?

You should be fine with the existing game files.
What you need to do is first install rockstar social club, then download and start the rockstar installer.
Once the installer has gone to about 5% pause it and close the installer, paste all the gamefiles to the installing folder.
Then reopen the installer, it should recognize the files and you shouldnt need to download the existing files anymore. However if you had an older version offline installer the rockstar installer will still download some updates.