30 years (fixed interest rate) 90% financing realistic?

It is a move-in ready purchase from the developer (i.e. including wallpaper, floors, etc.) "without any additional/hidden costs." Does the statement "it always gets more expensive" also apply here?

Not necessarily, but various small things might be missing, such as outdoor facilities. You only realize that afterwards. Or the standard version is so ugly that you "have to" upgrade at an overpriced cost.

Not necessarily, but maybe some small things are missing, like exterior landscaping. You only notice that afterwards. Or the standard version is so ugly that you "have" to upgrade at an overpriced rate.

Included are tiles that you can choose up to 35€/m^2. A parquet floor and painter's fleece throughout the entire house. The fittings/shower/bathtub etc. also looked quite good. Any concerns?

Actually, I gave wrong numbers above. It is a net 5000 without and 5400 with special payments calculated monthly to be able to cover the rate of 1950. This is at 39%, under the 40% that I have often read as the maximum recommendation. However, it is unclear to me whether this rough recommendation also applies to a 30-year financing.

Living in the Rhein-Main area is simply brutal. If we upsize, in any case at least 1500 EUR, actually 1700 EUR cold rent is to be expected.

Do the 400 EUR alleviate your mentioned worries? / Do you have experience that 39% rate can be maintained? (at least inflation helps out in the long run)

I would not do it in this form. 1) You are still young and your income will change significantly. The probability is very high that the current contract will not fit later and you will cancel it after 10 years. So why bear the additional costs of a 30-year fixed interest rate, especially if you are currently really tight and could use a significantly lower interest rate? 2) There are no children yet. This means that at some point the income will be lost for an uncertain period. Uncertain because you never know how healthy the child will be, how easy it will be with external care, etc. It is definitely quite a big risk. So either take the risk, but with another financing model or even better wait until the income has increased and the family planning is somewhat further along.

It is a ready-to-move-in purchase from the developer (i.e. including wallpaper, floors, etc.) "without further/hidden costs". Does the statement "it always gets more expensive" also apply here?

It applies just as much as the statement that you ALWAYS lose at the lottery. Of course, there are also winners. Expecting that there will be no additional costs is at least naive and, with a tight budget, negligent.

Included are tiles that you can choose up to €35/m². a parquet floor and painter's fleece throughout the whole house. The fixtures/shower/bathtub etc. also looked quite good. Any concerns?

It's all just super cheap: Tiles at €35/m² are, depending on the store, the lowest price category, which isn't even displayed. Definitely these will be small tiles with many joints. Not modern. Also such a herringbone parquet: extra cheap, rarely fitting the room. Not modern. But what do you expect from a developer? They install the cheapest of the cheap hoping that you will upgrade expensively. On the other hand, you ask about value retention and resale value. That just doesn't fit together. Oh, and what you see in show homes, brochures, etc.: That is all equipped to the maximum with chargeable extras.

It's all really cheap: Tiles for 35€/m² are, depending on the store, the lowest price category, which isn't even displayed. They will definitely be small tiles with many grout lines. Not modern.

Don't be so generalizing. Our 60x60 tiles for everything except bathrooms cost 25€/m². Of course, we could have chosen a similar grey tone for three times the price.