New construction or existing property? House or apartment?

You don't want to drive from Horb to Sindelfingen in the morning - that's annoying people...

Luckily, I don't have to (98% home office)

I find this an extremely sensible approach. A significant point in your life planning seems to be owning a house; personally, you seem flexible. In this respect, I would calmly choose "my" house and try to limit myself as little as possible by other external parameters. It is anyway the case that over time and as the children grow older, the social environment often changes or renews, and therefore a new start "somewhere" would in my opinion be a very good idea. I myself come from BaWü and have commuted daily by car and/or train virtually my whole life. Traffic has greatly increased, and in retrospect that was already a burden that today I might no longer want to take on. If it were a comfortable straightforward drive, maybe, but I would no longer want to push through morning/evening traffic jams. I was recently in BW again, and around Stuttgart the seemingly endless lines of vehicles streaming from everywhere caught my attention; when I was younger, I apparently had a different perception. An acquaintance of mine just sold a very old terraced house in Sindelfingen, and despite falling prices still extremely expensive; after renovation, the buyer will probably be around 1 million. If I don’t have to live there professionally or otherwise, why should I do it with all these disadvantages when I could fulfill my "house dream" somewhere else.

if only I knew whether you could trust the current providers!?

You can browse the subforums or use the search to see if it has already been a topic here. You can also directly ask about companies in the moderated subforum.

Or should one rather build independently, with architects and self-organization of the trades.

I think budget-wise that will be tight for you, as the plots in Eich are probably not bargains.

Do you think a townhouse for just under 550k € is realistically possible?

Don’t know. I don’t know your area personally. Sindelfingen on Immoscout already shows very expensive properties. If you move a bit into the surroundings, there are already houses that come into question, i.e. already renovated, not too old and overall rather solid. We once bought a townhouse over 20 years old. Affordable with a tiny plot. Only later after 14 years did we build on, for example, a leased plot. For us, that was more manageable than having to borrow 100,000€ for the plot. My tip: also look next door at what is offered there and don’t get too fixed. But it has to enable a generally rather comfortable life. It has to feel good. I rather don’t see that in a money pit. That might be tempting, but in terms of costs it is not calculable. And take your time.

I looked again yesterday evening at Immoscout within 30km around Sindelfingen, and there are several used properties under 500,000€ that are much better condition and also show a certain comfort/living value. Where you ultimately want to live is not clear to me now, you have to look for yourselves. If you are practically independent with 98% HO and her job, which offers a new flexible workplace in prospect, then a new start should be possible for you in a nice place.