If Online will make you more likely to play Red Dead games then I guess you are more into the multiplayer side of Red Dead than SP.
Regarding RDR, I'd not recommend buying the game, at all. I think its very overrated with a mediocre story and worst of all mindnumbingly boring content on offer. The wild west setting is simply used as background for the story. The open world itself feels a lot lifeless, and not the kind of wild west I had hoped for with much more research being put into making the setting very interesting to capture the time and place, but its just empty wasteland for the most part. No freedom within the tightly scripted missions. Pretty much like watching a movie than focused on creativity through gameplay. Its been a long time since I last played the game that I can't recall much about it.
All I can say is, it doesn't deserve all the praise it gets, in my opinion.
So, approach it with caution doing proper research before buying the game.
I will leave with these two helpful articles that sum up perfectly about the game's overall experience.
- RDR: Is It the Last-Gen's Biggest Zero? link
- RDR: Not that great link
Best to wait for RDR 2 to release and spend your money elsewhere for now.