Purchase of an existing single-family house prefabricated house year built 2000

So, from what I have read on the internet, I am allowed to install an oil heating system in any case if A I have no alternatives like gas etc., and B if I install solar thermal! In case an air-to-water heat pump does not work, I could also switch to a pellet heating system, which should cost around 10-20k!

A new oil heating system will probably be difficult; there are exceptions but not many. With 70kWh/sqm you should also be able to heat the place with a lower flow temperature, but that is not ideal.

I find the house itself quite expensive and would argue that with that budget you could also build 115sqm new. Then you would save most of the purchase incidental costs and have modern building technology.

I don’t find the offer cheap either! But it is almost impossible to get an affordable plot, having failed 3 times in the application process under the local resident model (one time we were even in the lottery). This has been going on for 2 years now; in hindsight, I could have bought a house for 800,000 two years ago and would have had almost the same costs as now! I was already close to saying we would stop trying to buy, but then when I see that for a rental house from the 80s they demand 1500 cold rent (renovated 2000), the savings are not that great compared to buying? Also, my bank advisor said, adjusted for inflation, one now has to expect 1800 cold rent! Now I have the dilemma, 1800 payment or 1500 rent! If rents really keep rising, I might be in 5 years financially as badly or as well off with renting as with buying!
As for the daycare spot, the village has its own daycare, and no one has been rejected there so far. Daycare costs about 250€, and since we live in Bavaria, there is another 250€ child education allowance from the state from the 13th month of life!

My plan would be, in case of pregnancy, to immediately change the tax classes from 4/4 to tax class 3 for my wife; to my knowledge, I have time until the 8th week of pregnancy for this... Since the parental allowance is calculated based on net income, we would thus have 150€ more per month in parental allowance! After the child is born, the tax classes will logically be swapped! That gets me to about 3300 net and my wife to about 750 parental allowance over 2 years plus child benefit... together that makes about 4300 net income in the 1st year and 4500 net in the 2nd year due to child-rearing allowance (there will be no back payments, I have calculated). From the 3rd year, my wife has to work again; there is no way around it! As a buffer, there is still the bonus amounting to 2000€ and tips averaging 300€, but I like to calculate without the tips, which is why they were not included initially!

So, from what I've read on the internet, I am allowed to install an oil heating system in any case if A I have no alternatives like gas, etc., and B if I install a solar thermal system! In case an air-water heat pump doesn't work, I could also switch to a pellet heating system, which should cost about 10-20k!

All alternatives to the heat pump will be discredited in the next few years to such an extent that in my opinion you shouldn't count on them. Even now, all alternatives are more expensive to operate, with a rising trend.

The problem with money and children is unfortunately often that a) things turn out differently and b) it happens sooner than you think.
Not all children settle in kindergarten without problems. Not all jobs can be done part-time.
And children unfortunately do not become cheaper the older they get. That means that from the (probably then part-time) job of one parent, far less is left than it would have been without a child.
I don't want to scare you, nor have I looked at your numbers in detail. But if you already have to factor in the tax class change (which, as far as I know, must be done a year before birth)...

All alternatives to WP will be so disparaged in the coming years that, in my opinion, you should not rely on them. Already, all alternatives are more expensive to operate, with a rising trend.

The problem with money and children is unfortunately often that a) things turn out differently and b) they happen sooner than you think. Not all children can be easily acclimated to kindergarten. Not all jobs can be done part-time. And unfortunately, children do not get cheaper as they get older. That means that from the (probably part-time) job of one parent, far less is left than there would have been without a child. I don't want to scare you, nor have I looked at your numbers in detail. But if you already have to factor in the tax class shift (which, to my knowledge, must be done a year before birth)...

The last 7 months are counted; I don't have to factor it in, but I also don't like to give anything away ;)

My plan would be to change the tax classes from 4/4 to tax class 3 for my wife immediately in case of pregnancy; as far as I know, I have time until the 8th week of pregnancy... Since parental allowance is calculated based on net income, we would thus have €150 more per month in parental allowance!

I'm too lazy to calculate now, but to reach the maximum amount you might have to switch already, since the parental allowance is calculated from the net income of the last 12 months.

I could also switch to a pellet heating system

Pellets need even more storage space than a stupid oil tank. Do you even have the space?
With 115m² living space, I would be grateful for every m² freed up to store stuff.

I assume you will have to buy solar thermal with a heat pump anyway, as you need shares of renewable energies. Alternatively photovoltaics, but the investment there is even higher.

I'm too lazy to do the math now, but to reach the maximum amount you might have to switch now, because the parental allowance is calculated from the net income of the last 12 months.

That's not entirely correct, a fictitious net income of the last 12 months is calculated. The basis is the gross salary, which is then taxed according to the tax class that applied in the last 7 months before birth. In case of doubt, one can also omit excluding the last month. But it is of course advisable to switch already at the beginning or when planning for children, then you can of course do it, because in the end, the tax return makes little to no difference.

However, switching back directly could lead to a tax payment, especially if you reverse the tax classes, so be careful!