One other thing I've heard about chromium oxide is that it's genotoxic, meaning it can cause mutation. It's also carcinogenic.
In the amounts
A very cheap stainless knife (or carbon if you have any carbon knives) would be good investments. The stones IMO are less important when beginning as
I think sandpaper is cheap initially, but if you look at knife ownership long-term I think you'll find sandpaper to become expensive compared to quality
I like my dmt extra coarse, but I don't have the more expensive diamond plates to compare to. One thing is that a finer dmt is nice to lap finer stones
Depends on the knife...I have some finger stones that are pretty fine that would do well for traditional knives, and they are pretty cheap.