Re: Which version to buy
The original games have been maintained by the community, and the downloads I linked to come pre-patched with upgrades that ensure they should work fine on modern OSes.
As for the Remasters, well, the only difference between the Steam and Origin version is that the Origin version does not have the integrated Steam Workshop, containing a ton of user made mods, maps and missions. Such addon content can still be used on the Origin version, but you'd have to find a different place to download it from.
