Financing plan -> Need for optimization?

Hello everyone,

My wife and I want to build a city villa and have a plot of land in sight.
Here are the key data:

Net:
Man 35 y.o.: €2300 (€2400 here, there is a salary conversion into the company pension scheme, so -€100)
Woman 35 y.o.: €1300 (with a 30h week)
1-year-old: €200 child benefit
Sum monthly income: €3800

No bonuses/annual salary increases/building allowance/tax refunds for a possible special repayment are considered here (also not a possible increase from 30 to 40 hours per week for my wife).

Plot including all costs: €115,000
Turnkey house: €285,000
Equity capital: €60,000

Through personal contribution, this sum will be reduced somewhat.

Loan offer:
€400,000 with €60,000 equity -> €340,000
At 0.65% for 15 years with a monthly rate of €1000,
the remaining debt would be €185,000.

We both have a home savings contract with 0,xx% interest at 7% repayment over €30/50,000 which will mature in 10 years, and we also want to use that to cover part of the remaining debt (then it would be paid off with a fixed amount of €330 over 15 years, a kind of securing the low interest rate). This costs us €200 over the next 10 years, which would then be "free" again.

That means we would still have to arrange €100,000 with a follow-up financing.
We would both be 50 years old then and want to be done with this at the latest by 65.

Our current calculation:

Insurances + GEZ and all the costs of a house including tele/internet/warm and a €150 reserve would be €550 per month + the €1000 repayment = €1600 monthly burden.

That would leave us €2200 for the month as of today.
According to our calculation of fixed costs + daycare + meal money etc., we would have €800 left for free disposal.

How do you evaluate all this and where do you possibly see errors or an estimate that is too tight?

Best regards
Mateo

Plot with all costs: €115,000
Turnkey house: €285,000
Equity: €60,000

Where is the rest of the calculation?
Additional building costs?
Outdoor facilities?
Buffer?

Additional construction costs for the land such as notary/tax etc. are already included in the land price. Outdoor facilities/garage will be created independently and offset against the savings from the own contributions (from the turnkey house). Here we can also manage without a carport or garage for the time being if it ultimately exceeds the budget.

There is actually no buffer, €20,000 on top would bring the remaining debt to €206,000, the rest would remain as usual.

Anything else?

well, I don't know if the construction budget is sufficient

That is why I am here and asking

Additional construction costs for the land, notary/taxes etc. are already included in the land price.

These are purchase incidentals, not additional construction costs.
What kind of turnkey house do you get for 285k? m² / features / special requests