Finance and buy or continue renting in the Stuttgart area?

Is the argument correct to opt for a fixed interest rate period of 15 to 20 years rather when building new? Honestly, I didn't quite understand the context. The advisor was mainly concerned about delays during the construction phase...

From when does the 10-year term apply? In my case: If I were to sign the purchase contract this month and the completion of the apartment would be next May, would the 10 years start from this month or from next year? The actual payment will probably only be triggered next year.

Hello everyone,

I have now taken care of my own calculation and, based on my perception, calculated what a flat could cost me at most as cold rent over a period of 10 years. To simplify the calculation, I assumed no increase in the property's value and opportunity costs of the invested capital at 5%. Feedback is welcome. It is also clear: if I ask the question in a stock forum, parameters will probably be set that portray a property worse. Here in this forum, probably the opposite. :) In the end, I have to decide for myself which parameters are realistic for me (with my own bias ;))

Total costs (including incidental costs and underground parking space) = 664,808 euros
Equity invested = 75,000 euros
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Loan amount 389,808 euros
Interest (10 years term) = 0.95% (with Dr. Klein calculator)
Repayment 3%

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Costs over 10 years
Interest costs = 30,591.77 euros
Incidental purchase costs = 30,408 euros
Reserves (1% of purchase price) = 41,000 euros
Opportunity costs (5% of 75,000 euros over 10 years) = 47,000 euros
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Total = 148,999.77 euros

Possible cold rent = 149,000 euros / 120 months = 1,240 euros / month

What do you think of the calculation? Any gross errors? The most debatable are certainly the reserves and opportunity costs.