I want to buy KSP but reading the reviews people are saying that the game has some kind of info stealing virus, is it true?

If i remember correctly, what you're talking about is something called Redshell, and it was removed years ago.
However, according to this post, the game now uses Unity's built-in tracking instead.
So if that concerns you (you should!), block the game in your firewall, or simply disconnect from the internet when you play.

Is it still online only? Can I opt-out of data collection?

I'm playing it since March and I had no problem, even when not connected to the web.