Ken, the link in your first post is for an 82 degree countersink. I suggest you purchase a 90 degree one for metric screws instead to ensure full contact with your M8 screw.
Keo, Weldon and MA Ford make some of the finest zero-flute countersinks in the industry. In fact, anything that MA Ford makes is top shelf but you can't go wrong with any of them. I highly recommend staying away from the Asian versions.
I've been using and sharpening these zero flute guys for a very long time. I use a hand drill, not a Dremel, but I use Dremel stones and sharpen from the back side of the hole. Conveniently, the inside of the hole acts as a nice guide to position and maintain the proper angle on the stone and allows you to sharpen the front cutting edge quickly and easily. You just keep the whole length of the stone against the inside of the hole and go around the cutting edges. Takes a few seconds to do and you have a sharp tool. I like the lavender colored Dremel stone, not sure of the grit. I use a hand drill or a Foredom - Dremels go too fast.
Zero flute countersinks actually run well at higher speeds. The small ones can be use at speeds of about half or more of normal drilling speeds. However, they last longer and cut just as well at low speeds, between 100 and 250 rpm.
When the hole is freshly drilled and gnarly in steel, with all sorts of hard stuff sticking up, I use a hand-held zero flute bit in a handle to knock those bits off before using a sharp countersink. Lasts longer that way.
For the single flute bits (I really like single flutes from MA Ford), use them at low speeds and feed hard enough to make sure they cut continuously. When I need a really nice countersink, like one that has to locate a tool, I use a sharp single flute bit from MA Ford. I have never had an MA Ford countersink chatter this way and the finish is excellent.
Multi-flute bits are horrible. I only use them in wood and plastics and only hand held in a handle.