Great link, Classic Motoring Accessories is a great place to shop. Very similar to the Eimann Fabrik Hi-Intensity Cleaner is Meguiars All Purpose Cleaner Plus which is what I use. Same principle, you dilute it to different strengths for different purposes. 1:1 is awesome for degreasing. I also use Meguiars Engine Kote afterwards. One of they key things is to NOT use high pressure as that will force water to go places where you don't want it. Let the cleaner do the work.
For tight places use toothbrushes. I also use my shop vac which can also blow to get most of the water off when I am done. Also, make sure the motor is either cold or to make the cleaner a little more effective warm it up a bit but, NEVER spray water on a HOT motor. Just to be safe I usually always do my cleaning when the motor is cold. I never let it get that dirty so warming it up is not necessary for me.
I rarely cover anything under the hood but, if you are paranoid cover anything you don't want water in. This is why I recommend no or very low pressure. Everything under the hood gets wet when you drive in the rain so it is not like there is anything under there that should never get wet. You just don't want to use high pressure and force water to go places it normally wouldn't.