So with your calculation of 270k including the land, I would be cautious, we are also just about to finalize the house construction contract and with not quite 10m² more we come to 260 euros without land. The house itself also "only" cost 180k-190k, special requests listed below together another 30k, incidental construction costs with supply lines (electricity, water, sewage, telecom) + soil replacement again nearly 40k + kitchen and suddenly we were at 260k.
Of course, 20-30k of that was luxury equipment and of course you can install a used kitchen for a few euros but don’t underestimate the 180+30k construction incidental costs.
With the turnkey offer, many things are still not included that you might want, for example
-[Wärmepumpe statt gasheizung(mit Fußbodenheizung)]
-[KFW70 ?]
-[Lüftungsanlage mit wärmerückgewinnung?]
-[Austausch von Boden falls dieser nicht zum hinterfüllen geeignet ist]
-[Küche?]
-[Andere Möbel? z.B. Lampen, Schränke, Garderobe....]
-[Ebenerdige Dusche statt normaler evtl. 2 Dusche im Gäste-WC]
-[Verkabelung für Sat / Netzwerk]
-[.....]