75-250 is indeed a pretty large range. But not too unusual to do that. According to https://www.ysi.com/water-hardness that would be somewhere between soft and slightly hard (no particular endorsement; this was just the first site that I found online). I don't think you'd go horribly wrong by telling your oven to treat this as "medium". Worst case, it prompts you to descale the unit more frequently than strictly necessary. That'll waste a little money on supplies. But if in doubt, that's better than wasting money on repairs. I recommend you buy the descaling tabs in bulk. They regularly show up on Amazon and other online services. And they definitely get cheaper if you buy a bigger package. You'll need two per descale cycle. I usually need to do so every six to nine months. But that might not be the same for you. We use the steam oven very frequently, but we have very soft water. Also, you absolutely want to make sure you have tabs on hand, when the unit needs it. You don't want to find yourself in a situation where you can't use the oven for a few days while waiting for supplies.

First of, congratulations on your good deals for these three units. They will serve you well. For ventilation and wiring purpose, you will need a gap at the back of the shelf behind unit. For combi steam oven, this is where the oven sucks in the air in. It will vent warm air to the front underneath fascia. So following diagram #1 looks fine. For the sentence "The cabinet housing must not have a back panel fitted behind the installation niche", this must be from a newer manual of the combi steam oven. For example, see newer 6805 manual vs older 6805 manual. In a version that I have it uses "Make sure there is no back wall behind the building-in recess" which means that there should not be a cabinet back wall right at the cut out area. See picture for air intake in combi steam manual in installation section. In terms of installation each is 18" tall, just need to be arranged such that coffee machine is not too tall, and speed oven not too low. Perhaps somewhere where the bottom of the combi steam oven is a little bit below bench top level. A combi venting warm air at the front is not an issue for comfort. I have only combi steam oven in my kitchen and its bottom is at the bench top level which is very convenience, but that would put coffee machine a little too high up, if it is going to be on top. Here is an image from mile-vs-wolf-steam-oven thread. Found a funny image on the web of Miele ovens, noted the coffee machine location.

This does sound plausible. And yes, in general, forced convection is a good thing. You always have some convection anyway. That's one of the three modes of how heat gets transferred -- and it's probably one of the major ones in an oven. Natural convection just happens to be very uneven. Forced convection can help with that, if the oven is designed to move air around evenly (not all convection ovens get this right; I think the Miele CSO is usually pretty good, though). Also, both forced convection and increased humidity both have the benefit of much faster heat exchange. That's how you get faster cooking/baking times in a CSO compared to a traditional oven. This is usually what people want. But it might require you to adjust your recipes for either shorter times or for lower temperatures. In ovens that rely on natural convection, I have found I always inevitably have to insert extra heat dispersers (aka "a unused cookie sheet on the bottom shelf") to bake more evenly. With Miele's model, that's less of a concern. I am not overly concerned about Miele running the fan at all times, and so far I haven't had it be a problem. What exactly did you try that didn't work? Maybe, we can help you figure out how you need to adjust things. Also, do you know whether Miele runs the fan at full speed when in non-convection mode? It would make sense to reduce the fan speed for these modes.

Our Miele CSO is almost 6 years old. Wonderful piece of engineering! Great for all kinds of food...for baking and cooking. We chose the Miele over the Wolf because it was the preferred choice of this forum (the old Gardenweb). Posters were pretty much universal in saying that the Miele is easier to use, and much easier to clean. Also, we were pairing the CSO with a regular convection oven. The Wolf ovens had a problem with the blue enamel chipping. Not sure if that situation has been solved.