In Europe:
The measurements for Runway Visual Range (which is different to Met Viz) are the numbers used for Lo-Viz approaches. Generally there are Touchdown, Mid-piont and Stop-end recordings so Air traffic will pass a figure such as "400, 300, 350" These are RVR readings in metres taken automatically at about a metre above ground level level by machines.
To answer your question about Viz being different at decision rather than on the runway, that is the whole point of RVR readings being measured, as it is the runway you have to land on and (for Cat1 and II) therefore have to be able to see. So whilst you may be able to see the runway fine from overhead or even 1000' it may be a pea-souper at runway level and RVRs tell you this.
The requirements for approaches are generally 550M for Cat 1, 300M for CatII and anything down to 0M for CatIII depending on the aircrafts and airline's rating A,Bor C. These RVRs can be increased to much higher figures for various reasons (terrain, lighting systems) on particular approaches.
I don't know anything about HUD except they look very fun.