So I'll give you a few tips to manage with the tight budget:
- simple architecture, no corner windows or protrusions, balconies
- no basement
- no ventilation system
- as little insulation on the house as possible, no floor slab insulation
- no ceiling spotlights, normal lamps
- small kitchen without island with exhaust to the outside
- build the house not completely airtight
- gas heating
- no additional heat source like a wood stove
- plastic windows with shutters
- garden only the essentials, no fence
- no double garage, preferably a carport or parking space
- electrical system only sockets and switches. No LAN or smart home
- bathroom fittings you have to get from the internet or similar
If you implement most of this, then perhaps you can manage with the 300,000 euros without land somehow. But additional costs definitely still apply.