After weeks of trying I finally managed to buy an
IBM 5150 on EBay (yes, that's the first IBM PC). EUR 100 plus EUR 70 for shipping - but hey, it's worth the price.
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I found original IBM DOS 2.0 disks as well, which complete the whole purchase (IBM DOS 1.1 or 1.0 would have been even better, but they are also more rare).