I bought the original Metro 2033, but it was basically unplayable for me due to the gawdawful mouse acceleration (i.e. horizontal and vertical mouse moved at different speeds).

Is that still an issue with the Redux version?

There is a mod that removes mouse acceleration and the slower vertical mouse sensitivity.
https://community.pcgamingwiki.com/files/file/966-metro-mouse-fix/

I forgot how but I fixed it and it works just fine. I fail to see why the game needed a redux version. I am also on the original version. Some have said the stealth is a tad too hard but not having completed the game yet, I cannot comment on that.