What kind of knowledge is required to do a PhD?
I think everything is about your motivation. You need a subject you're very interested in and of course you don't know everything yet you need to do the job. This is part of your work. In the end you’re getting paid to teach yourself 


  but none of them is something I would consider researching on    


This is a bit strange and dangerous. You’ll be working on a field for multiple years and you should choose an area you really really like. Imagine you're forced to do stuff every day you don't really like... You'll be demotivated and quit your research.

However, I think your future research should at least somehow be related to the stuff you do during your studies. Otherwise you have no clue what you're going to do or how your daily work will be.

Maybe you should talk to other PhD candidates from a group you might want to join after you studies - just to see how their daily work looks like and whether this might be the stuff you want to do for the next years.