Sorry , that I still have to write something here, but this issue is really burning under my nails...
The OP clearly wrote that he does not want relationship advice here... totally understandable and repeated as well. Why can't some people here respect that?
1. He wants a house now
2. but his girlfriend does not want to get married yet
What an entry 50:50 in the land register means considering the gift tax issue, you experienced homebuilders don't need me to explain now (could be arranged via a private contract/interest-free loan, but the OP prefers to finance the house alone - period)
The hint that he should rather wait until marriage is nice - but see points 1 and 2.
and 3. maybe the girlfriend also does not want half a house on her back yet?
And again - the super great hint about the "imminent" parental support - related to the situation, taking into account the legally anchored retention and exemption amounts would have been acceptable - but just throwing the horror scenario "all the money is gone" - well...
So and now to finish "what I would do":
1. With 350k in the back, I would look for a monthly rate I can/want to pay
2. then I would check how much credit I can get for that
-> playing here with different equity contributions and watching how the interest rates move
3. that leads to different budgets based on the different equity contributions
4. find out what you can build with that and decide...
So, I would probably end up with a more modest number than you plan, since I would want to keep a nice part of the equity for the following reasons:
- with the excellent interest rate, I would invest the retained equity so that hardly/no loss or even a profit occurs
- separation -> pay installment + full additional costs alone
- possibly self-employment
- wedding, travel, pilot/sailing license, whatever...
So, I'm out here now...