Right from the start, you will have massive difficulties finding a bank that will finance this properly. Fixed-term contracts are a problem, and your fixed net income is not high enough to finance €300,000. I gather from the article that you are not married. This makes the whole thing personally more complicated; the bank doesn't care about that.
If you are sure that you fit together, ask your families what they think of your plan. Maybe help can be provided here, and your equity share increased. Otherwise, I would advise you to save diligently for another 3-5 years. This will probably not be a bad plan. In 5 years, the first "bad" loans will expire, and many will realize that they may no longer be able to service the new interest rates. Eventually, interest rates will have to go up again. When that happens, there will be a lot of 10-year-old properties on the market. Maybe then there will be something for you, and your family and financial situation will be clearer.
Another alternative is to move further out. If plots start at €250 in your area, then these are also the plots that no one wanted before. The really good plots will probably cost more. We had the same problem. In Karlsruhe itself prices start at €500 and in the "Speckgürtel" (commuter belt) at €350. However, transport connections are catastrophic. We have now moved to the Pfalz... 25 km away. I need about 25-30 minutes to get to work in the morning, and it would take me 60 minutes by public transport. So acceptable, considering that I used to take 25-35 minutes to get to work from downtown, whether by car or public transport. And that was only 9 km :) Our plot price was then €120/sqm... still high, but affordable.
You also have to stop seeing everything as one sum. You have to break down the costs so that they can be evaluated. Because in the end, something must remain for the house, your total sum. Assuming a minimum of 400 sqm, the plot will cost you from €110,000. Plus €35,000 ancillary building costs. For a small house (approx. 120 sqm) without a basement, you will have to pay at least €200,000, no matter what the offer sheet says. It simply can't go much lower unless quality suffers.