How much free speech should there be before government intervenes, e.g. denying the Holocaust, racist language.
I kind a like how that is being addressed here. Offensive things can be said in comedy, in art, in debates, but, when they are used to spread and organise hatred, they are forbidden. I know the line is thin, but, it gives a freedom to debate and such, where it also gives an option to act against it when it is used in a damaging way. Now it is possible to discuss risk groups in crime or illnesses, without being prosecuted for racism, sexism or what ever.
Does someone have the right to buy harmful items/goods, even if it will have a bad effect on him/her?
Depends, in general, yes. In particular, no, not always.
Everything can be harmful, when used wrong. A rope can be used for skipping and for hanging oneself. Selling ropes with noose and all, for hanging, that should not be possible, IMO.
Is paying a tax an assault on personal liberty?
Yes, it is, but so are the things paid with the income of said taxes, and most people do see things paid with said taxes as a right, and, when it comes to roads and such, as something enhancing their liberty. So, it is an assault on personal liberty, but, it is not a bad thing per se.