So.... nearly another year later I see interest in this thread - sorry, I didn't get an email notification about it. I'm stumbling across it again more or less by chance now.
My house is standing, everything's fine - and here is the experience:
(1) You can safely do without people like Bauexperte. Know-it-alls and schoolmasters who can't/won't help you (even if you are only looking for a rough orientation) should just hold their breath (or keep their fingers off the keyboard).
(2) Soil reports are a thing... sometimes they bring very useful information, sometimes it's just overkill - like using a cannon to kill a sparrow - and sometimes two meters to the left or right make all the difference. So a soil report is not the holy bible when it comes to building a basement. For me, it would definitely have been over the top. Since I built in a known residential area with older and newer neighboring houses where the (soil) conditions were not entirely unknown (location of rivers, approximate groundwater level, soil consistency from direct neighbors' experience, etc.) one could estimate with a good conscience that there would be no nasty surprises. So: definitely not money wasted in the wrong place.
And what REALLY happens in the ground, you only see when you start digging. No localized soil report helps with that. Don't let yourselves be unsettled by anyone who thinks building without a soil report is "reckless" or by others who got ripped off by their builder through panic-mongering or the like. Go by your own sense of the situation, the ground beneath our feet is not some nebulous monster you need nightmares about.
3) Quanta Costa in my case was in the lower four-figure range (I don’t have the exact amount at hand right now, sorry ><) Mind you: I had a river just across the (village) street where I was allowed to drain! That surely helped a lot & saved money!! Because the pump ran constantly day and night until the basement was finished.