For me, all of this sounds more than awkward. It would simply be too much money for me to weave such a hard-to-understand web. I don't think anyone can really help you here because, based on the text alone, without lawyer-reviewed contracts, you can interpret anything and nothing into this deal. For me, it’s a clear reason to stay away from it.
Yes, we are also unsure. Only, if the demolition were already there now and it simply wouldn’t be sold along (which is basically the same), that would normally be fine—like a semi-detached house, for example.
The construction is actually quite easy to understand. We rent first, then we buy, and possibly there will be an extension at some point. However, I didn’t understand the seller’s motives, for example, he is renovating the house (having it renovated) for well over 300,000 and we get to have a say.
One could naïvely think it might just be that he cares about the property and wants sensible people living in it… We know there were offers (significantly higher) that were rejected because those wanted to tear down the property. It was clearly stated that no speculators are allowed to buy.