Dhoom -
GoG (Good Old Games) got its start as an online direct download vendor selling classic games that they brought up-to-date and provided DRM free. DRM free is their mantra. The do have contracts with some Publishers for newer games like the Witcher series and some Indie stuff, but most of their stuff is classic games going way back to more recent like Civ IV. GOG is a very reputable vendor.
They sell digital versions of games. (Disk versions of PC games are basically obsolete at this point my friend) Unlike Steam, you do not need a client to download/play games. GOG does have a client called Galaxy that makes downloading/updating games easier/faster, but it is not required to play/download the games. You will want to create an online GOG account to purchase games there. That way you have a library on your account and always have the game to download whenever/wherever.
You might enjoy perusing the GOG catalog. Some great old games on there (I recommend Desperados: Wanted Dead or Alive and Silent Storm)
https://www.gog.com/
I don't have the GOG version of Civ IV, but it appears to be the Complete Edition. I would assume that it is like any other digital version of the Complete edition, with the exception of Steam's which has a different DLL. So, in short, it should be the original format and work fine with BUFFY out the gate, unlike Steam's.
(I did get a copy of the Steam version myself, as it doesn't have any issues with Win 10...it does have issues with BUFFY but there is a workaround..albeit a bit painful)
Don't you have a Steam account now?
Oh..and keep checking GOG out for IV on sale. They have big sales every week.